fix: upstream key config add mqtt_client_id support

[AlinsRan]

Description

Fixes #9449

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)

[monkeyDluffy6017]

There are many variables and we cannot list them all.
how about rewrite like this: https://github.com/apache/apisix/blob/master/apisix/schema_def.lua#L454

[AlinsRan]

There are many variables and we cannot list them all. how about rewrite like this: https://github.com/apache/apisix/blob/master/apisix/schema_def.lua#L454

I think constraints are necessary because they can help users follow the best practices.
Under constraints, many situations are predictable.
I believe that flexibility does not lie in having more options, but in providing better options.