fix(api7): mismatched fields in upstream mtls

[bzp2010]

Description

API7 let the upstream mtls settings as cert and key instead of client_cert and client_key in the converter, which is not schema compliant, so fix it.

Also, cert and key do not emphasize their roles, so they need to be modified.

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: This aligns the schema so it's a fix and not backward compatible.