provider/google: stop trying to set mysqlReplicaConfiguration on read

[danawillow]

This is branched off of #14336, so that should be reviewed first.

Fixes #11651.

Even though it's part of the API, mysqlReplicaConfiguration is never returned on read. The documentation notes:

MySQL specific configuration when replicating from a MySQL on-premises master. Replication configuration information such as the username, password, certificates, and keys are not stored in the instance metadata. The configuration information is used only to set up the replication connection and is stored by MySQL in a file named master.info in the data directory.

Because it isn't returned on read, trying to read after setting it on create would crash every time. This fixes that crash.

This change also sets MaxItems: 1 on replica_configuration and removes read-time checks that it only has 1 item.

[catsby]

LGTM pending the prerequisite PR

[ghost]

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