feat(ai-proxy): add support for pushing logs in ai-proxy plugins

[Revolyssup]

Defines log format and add support for pushing ai-proxy request/response in logs to any logger.

The information about logging is already in the doc https://github.com/apache/apisix/blob/master/docs/en/latest/plugins/ai-proxy.md

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)