feat: consumer key duplication check

[Baoyuantop]

Description

This PR adds a duplicate consumer key check when creating consumers and credentials using the admin api to prevent different consumers from having the same key.

Fixes #11197

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)