[fix] Add resource parameter to pushed authorization code request

[srosenda]

Description of change

The OpenID4VCI specification RECOMMENDs that resource parameter is added to authorization request when _"Credential Issuer metadata contains an authorizationservers property":

If the Credential Issuer metadata contains an authorization_servers property, it is RECOMMENDED to use a resource parameter [RFC8707] whose value is the Credential Issuer's identifier value to allow the Authorization Server to differentiate Credential Issuers.

See https://openid.net/specs/openid-4-verifiable-credential-issuance-1_0.html#section-5.1.2-9.

This PR sets the resource property to the credential issuer identifier (URL) when credential issuer metadata has authorization_servers property and also changes the issuer metadata authorization_servers property to OPTIONAL as specified, see https://openid.net/specs/openid-4-verifiable-credential-issuance-1_0.html#section-11.2.3-2.2.

Fixes #57.

Type of change

Please delete options that are not relevant.

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• [x] Tests in this package pass (except for tests that have been failing in the main branch since https://github.com/eu-digital-identity-wallet/eudi-lib-ios-openid4vci-swift/commit/054a2b2e484ae25f04fe3d1113b36d6a7fedf4a4)
• [x] Works against the Finnish EUDIWallet backend that requires the resource parameter to be present in authorization requests.

Checklist:

• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the readme
• [x] My changes generate no new warnings
• [ ] I have added unit tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes (except for tests that have been failing in the main branch since https://github.com/eu-digital-identity-wallet/eudi-lib-ios-openid4vci-swift/commit/054a2b2e484ae25f04fe3d1113b36d6a7fedf4a4)