Flasksimpleauth

Add support for `| None` typing (Python 3.10+).

- better doc
- add `FSA_HANDLE_ALL_ERRORS` directive

- improved documentation
- use `application/json` instead of `text/json`

- improved documentation
- `FSA_DEBUG` changed to `FSA_MODE` with upward compatibility
- show µs in *dev* mode

- improved documentation, shown on `github.io`.
- add `error_response` and `FSA_ERROR_RESPONSE` to control error responses.
- add `add_headers` and `FSA_ADD_HEADERS` to add headers to responses.
- add `FSA_BEFORE_REQUEST` and `FSA_AFTER_REQUEST` to provide some from the configuration.

- improve fix around early return
- better doc and tests