Libindy allows issuing of partially filled credentials

[Artemkaaas]

Libindy 1.15.0 allows us to issue a credential containing only part of fields specified in a credential definition/schema.
But next a proof generation with the usage of that credential fails with:
`
TRACE:indy.libindy.native.indy.api.anoncreds: src/api/anoncreds.rs:2050 | prepare_result_1: >>> Err(IndyError { inner:

UrsaCryptoError: Credential doesn't correspond to credential schema

Invalid structure })
`

Libindy/URSA must validate that credential attributes fully match to credential definition / schema during the issuance.

Sample reproducing the issue can be found here: https://github.com/Artemkaaas/indy-sdk/blob/bugfix/partial-credential/samples/python/src/anoncreds.py