R2DBC connection property names should match the JDBC ones

[ttosta-google]

Currently the doc r2dbc.md does not specify that the options can be added to the URL. However it can and works.

Example:

r2dbc:gcp:mysql://<DB_USER>:<DB_PASS>@<CLOUD_SQL_CONNECTION_NAME>/<DATABASE_NAME>?TARGET_PRINCIPAL=<IAM_EMAIL>

To make the connection property names match the JDBC ones, we should update GcpConnectionFactoryProvider.java:

• unixSocketPath
• ipTypes
• cloudSqlDelegates
• cloudSqlTargetPrincipal
• enableIamAuth

Also update the doc r2dbc.md.

Note: we need to support both the old options TARGET_PRINCIPAL and the common options cloudSqlTargetPrincipal just in case some customers figured out how to use TARGET_PRINCIPAL in the r2dbc url.