Send User-Agent header on registry requests

[Rohithmatham12]

Summary

• add a default User-Agent header to RegistryClient requests using the existing clientID value
• keep an explicitly supplied User-Agent header when callers provide one
• cover default and explicit User-Agent header behavior in registry client tests

Fixes apple/container#1583.

Testing

• git diff --check
• swift test --filter requestHeaders (blocked locally: package requires Swift tools 6.2.0, installed Swift is 6.0.3)