feat: integrate UCAN revocation handling in access-client

[pranavkonde]

Enhanced the access-client package to better handle UCAN revocations by adding dedicated methods for fetching and validating revocations. The changes improve security by making revocation handling more robust and easier to use.

Key changes:

• Add getRevocations method to Agent class for fetching revocations
• Add validateDelegation method for revocation validation
• Add RevocationsStorage interface and types
• Integrate with existing validation flow from upload-api
• Add proper error handling for revocation scenarios