Aries-staticagent

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- Various doc and improvements as a result of static analysis and other code
quality checkers
- Streamlined examples to more clearly demonstrate library
- Rework "replies" concept into "sessions".
- Add Keys and Target for encapsulating connection information. Simplifies
`StaticConnection` by delegating connection info tracking to these classes.
- Add `preprocess` decorator and specializations for validation and MTC
decorators.
- Update types in examples to use didcomm.org message types.
- Use didcomm.org forward message type.

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Hotfix Patch
- Accept Base64URL encoded values with and without padding.

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Highlights
- Improved standard message dispatch bypass mechanism.
- Improved interface for MTCs.
- Significantly increased code coverage in unit tests resulting in overall
improvement in reliability.

Detailed Changes

General Fixes and Improvements
- Export `MessageDeliveryError` from package root.
- Fixes for forwarding and processing forwarded messages.
- Fix Message equality checks.

Message Trust Contexts
- Add MTC helper functions for checking and setting typical message
characteristics.
- Refactor `Additional Data`; now a class instead of a dictionary. Currently
only has `sender` and `recipient` as properties. `sender` and `recipient` are
also accessible as properties on MTC.

`StaticConnection`
- Remove `ConditionallyAwaitFutureMessage` construct in favor of simpler and
more robust `connection.next` method.
- Refactor send method into injectable dependency. Defaults to HTTP.
- `unpack` will take a dictionary or bytes as input.
- `pack` takes `plaintext` as an argument.
- Add `next` context manager method, yielding a future to the next message
matching the given type or condition. This mechanism is now also utilized in
`await_message`.

`Utils`
- Move HTTP sending code to method under `utils`.

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Fixes
- Fixed preparation of `Forward` message (PR 26)

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Highlights

- **Add support for forwarding** for connections utilizing mediators for
routing.
- **Simplified `StaticConnection` and added mechanism for updating "their"
info**, resulting in differences in the method signature of its constructor.

Detailed Changes

`StaticConnection`
- `__init__` - changed to accept tuple of keys (public, private) with
optional named arguments: `endpoint`, `their_vk`, `recipients`, and
`routing_keys`.
- `endpoint` - sets the endpoint to which messages are sent.
- `their_vk` - mutually exclusive with `recipients` and results in a
recipients list of size one containing `their_vk`.
- `recipients` - mutually exclusive with `their_vk`, setting the contents of
the recipients list for this connection.
- `routing_keys` - list of mediator keys that will forward the message on to
the recipient(s).
- `pack` prepares messages for forwarding when `routing_keys` is set on the
connection.
- Renamed convenience accessors for keys: `my_vk -> verkey`, `my_sk -> sigkey`.
- Added `update` - Updates "their" connection information for this static
connection. Arguments are named arguments; processing is similar to
`__init__` with the exclusion of "my" keys. A change in my information should
be viewed as creating a new connection and therefore updating those values is
not supported.

Crypto
- `pack_message` - added named optional argument `dump` that will convert the
packed message to bytes when set to `True` or will leave as a dictionary
when set to `False`. Defaults to `True`.
- `bytes_to_b58` and `b58_to_bytes` - added a small (max size of 16) `lru_cache`
to make frequent conversions of keys in the pack, unpack, and message
handling process more efficient.

Utils
- Added `ensure_key_bytes` - ensures that the given key is formatted as a
byte string rather than a Base58 encoded string.
- Added `ensure_key_b58` - ensures that the given key is formatted as a
Base58 encoded string rather than as bytes.
- Added `forward_msg` - creates a new forward message.

Examples
- Updated to reflect new method signatures.

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Bug fixes
- Fix condition not passed to `await_message` inside of `send_and_await_reply`.
- Return signer on `verify_signed_message_field` so that the signer of the
message can be verified to be the expected signer.
- Fix usage of removed `_key_for_type` in dispatcher
- Fix tests

Additions
- Added more connection information to `StaticConnection`, including: `did` as
Base58 encoding of first sixteen bytes of `my_vk`, `my_vk_b58`,
`their_vk_b58`.