✨ Add VerifyCredential query

[ccamel]

Context

An axone-vc authority must expose a lightweight way to check whether a credential can be used without returning the serialized credential payload.

Expected behavior

Add a VerifyCredential query.

VerifyCredential {
    identifier: Uri,
    valid_at: Option<Timestamp>,
}

The query must return whether the credential exists in the authority's active credential set.

If valid_at is provided, the query must also evaluate the credential validity interval.

Unknown identifiers must not fail the query.

Validity

For valid_at:

• valid_from is inclusive when present.
• valid_until is exclusive when present.
• missing valid_from means no lower bound.
• missing valid_until means no upper bound.

A revoked credential is considered unknown because revocation removes it from the active credential set.

Response

Expected shape:

VerifyCredentialResponse {
    exists: bool,
    valid: bool,
}

Where:

• exists is true when the credential is present in the active credential set.
• valid is true when the credential exists and matches the requested validity check.

If valid_at is omitted, valid must be equal to exists.