feat: support anonymous consumer

[Revolyssup]

Description

Fixes # (issue)
Currently, the built-in user authentication logic in APISIX is either successful authentication reflected to the corresponding Consumer object, or authentication failure directly returned to the caller authentication failure return.

Some users want a dummy consumer framed as "anonymous" consumer to be attached to the request when authentication fails so that certain rules (as plugins) defined in the "anonymous" consumer will be applied.

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)