Varvault

Made some bugfixes related to resources, where creating a resource that had a relative path to the same directory was not working. It was also not possible to create a vault without supplying a resource which it absolutely should be possible to do.

Added new functionality to keys, where the usage of a certain key is logged in the key itself. It only logs usage in functions decorated with `vaulter` for now, but it can be quite helpful when you need to understand why a key is missing in the vault, or if you just want to know where a certain key is used if you are working on a big project.

Fixed a bug related to live-update and vault-structs which meant that specialized keys would not be reloaded as the correct key type. Added a test for this. Also made some fixes to how the state for a resource is handled. Simplified `create_mv_from_resource` to just take the expected keys as an argument.

New features:
* Added the possibility to define input keys through the name of the parameters in the signature of the function. It essentially comes down to calling a parameter the same as the name of a key in the keyring and making sure the flag `VaultFlags.use_signature_for_input_keys` is defined. This can reduce boilerplate, but does make it harder to track the usage of certain keys, which is why it will never be the default behavior of varvault.

Other changes:
* Updated the conclusion section of the README.md.
* Reworked the filehandlers API and simplified the JsonFileHandler slightly.
* The filehandler now holds the state and keeps track of it, rather than varvault itself keeping track of the state.
* Made changes to the vaultstructs API to change the name of the abstract method to `create` which is a lot less cumbersome of a function name.
* Made significant cleanup of code, mostly in vault.py to make it easier to work with, and to read and comprehend.
* Renamed filehandler.py to resource.py as that is more what it is than a filehandler, even if that will most likely be what it will be used for.

Made bugfixes for both live-update and filehandlers, which prompted some expansion to the API for the builtin `JsonFileHandler`:
Added a new flag `create_file_on_live_update` to the `JsonFileHandler` which essentially creates the file when creating a filehandler that is meant to work with live-update because sometimes you want to create a new vault that is meant to fetch live-update changes later.

Added some messages to all asserts in the code that lacked a message to make sure the errors are described in more detail.

Made a fix to `input_key_can_be_missing` when using the `vaulter` decorator. Due to a recent change in `get_multiple`, this had to be changed as well so it works as expected. The user still has to make sure the input key that is missing is handled correctly in the decorated function.