feat(signer): add direct COSE callback binding

[s35-lee]

Changes in this pull request

Adds Signer.from_cose_callback(callback, reserve_size) for external signing services that receive serialized C2PA claim bytes and return a complete tagged COSE_Sign1 structure.

• Registers the c2pa_signer_create_direct_cose native binding.
• Validates reservation size, callback result type, and exact output length.
• Reports callback exceptions and invalid results as native signing failures.
• Retains the callback through signer lifetime and Context ownership transfer.
• Frees the native signer if Python instance construction fails.
• Adds direct-COSE tests to the existing test_unit_tests.py suite.

This PR has dependency on contentauth/c2pa-rs#2848

Before merging and releasing, c2pa-native-version.txt must reference a native release containing c2pa_signer_create_direct_cose. The current version pin remains unchanged; local validation used C2PA_LIBRARY_NAME to load the new library.

Checklist

• This PR represents a single feature, fix, or change.
• All applicable changes have been documented.
• Any TO DO items (or similar) have been entered as GitHub issues and the link to that issue has been included in a comment.