Description

1. Add a new metadata response header to the Standalone API. This includes X-Last-Modified and X-Digest.
   They are used to report to a cluster of multiple APISIX instances, which instance was last updated, and the configuration digest currently in use.
   The X-Digest is passed in by the PUT request and recorded in memory, and then output in the response header of a GET or HEAD request. This value is transparent to APISIX, which does not parse or generate the value in any way; it is completely controlled by the client. There is a simple judgment here; if the digest input for the PUT request matches what is currently stored in memory, the API will return 204, and the configuration update will not actually occur.

2. Add support for the HEAD method, which returns only metadata but not configuration.

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)

I explicitly point out that this is not backwards compatible. Since this feature is clearly marked as experimental and users have been warned not to rely on it, we can comfortably make the changes we need.