feat: implement credential

[dspo]

Description

This PR add a resource named credential and implement Admin APIs for it.
Credential is used to hold the authentication credentials for the Consumer.
Credentials are used when multiple credentials need to be configured for a Consumer.

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)

[nic-6443]

@kayx23 Please help review the documentation part of this PR.

[moonming]

Is this a break change for using the authentication plugin on consumers?

[dspo]

Is this a break change for using the authentication plugin on consumers?

It's not a break change. Users can still use Consumer in the same way as before, and the one change to Consumer is that it is no longer required to have at least one auth plugin.

[kayx23]

@kayx23 Please help review the documentation part of this PR.

No additional comments from me for docs.

[kayx23]

doc lgtm