simplify client extension to work with not-quite-compliant servers

[hauntingEcho]

As SCIM2 is a relatively young specification, many implementations still have quirks which must be worked around. Removing the 'final' modifier from the builder classes and allowing protected access to the relevant methods greatly makes it much simpler to keep a client compatible with many servers by extending the builders with the necessary workarounds.

I've opened PR #101 to highlight the particular methods giving me trouble, but there may be more which could benefit from this.

[hauntingEcho]

This is sorta related to #81