Add POSTGRES_CLIENT_VERSION option

[abordage]

Motivation and Context

Enabling WORKSPACE_INSTALL_PG_CLIENT or PHP_FPM_INSTALL_PG_CLIENT will install postgresql-client-12, regardless of which PostgreSQL version you choose. This PR allows you to specify the version of postgresql-client

Types of Changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).

Definition of Done Checklist:

• I've read the Contribution Guide.
• I've updated the documentation. (refer to this for how to do so).
• I enjoyed my time contributing and making developer's life easier :)