Improve submission validation and input extraction.

[timothee-haudebourg]

This PR improves the presentation submission validation in two ways:

• Input extraction: currently we are only doing validation. But I also needed to extract the inputs provided in the submission. I added a PresentationSubmission::find_inputs function to find and extract the inputs. It returns a MatchingInputs structure containing all the inputs that matched the presentation definition.
• Input decoding: currently the presentation submission inputs are not decoded, which is required to correctly handle nested descriptor maps. I've added a ClaimsDecoder trait that is given to the find_inputs to decode the visited input claims. I provided a "default" implementation NoClaimsDecoder that doesn't decode anything (so it behaves exactly like the current implementation).

For the validation part:

• I added a MatchingInput::validate to validate the extracted input against the presentation definition requirements.
• I replaced the previous validation function with PresentationSubmission::find_and_validate_inputs that just calls find_inputs and validate.

Other changes

I've set some fields public and removed some accessors.

[Ryanmtate]

Nice work @timothee-haudebourg!