[Breaking] Replace strings with enums where ever applicable.

[anekkanti]

This a breaking change, but is backward wire compatible.
Meaning the existing clients will keep working when this change is deployed on the server
But the consumers will see compile time errors when they pull the change.

String fields being replaced with enums:

• Namespace::state
• User::state
• ServivceAccount::state
• ApiKey:state
• AsyncOperation::state
• NamespaceRegionStatus::state
• AccountAccess::role
• NamespaceAccess::permission
• ApiKeySpec::owner_type
• NamespaceSpec::custom_search_attributes

Replace string (base64 encoded pem certs) ca certs with a bytes field (pem certs):

• NamespaceSpec::mtls_auth::accepted_client_ca