wafsamba: introduce SAMBA[3]_PLUGIN()

wafsamba: introduce SAMBA[3]_PLUGIN()

This will be used to define plugins we provide to be used
via dbopen/dlsym to external consumers.

SAMBA_PLUGIN() is used instead of SAMBA_LIBRARY() in order
to make it more strict that these plugins can't be used as
normal depedency by other subsystems and libraries.

With require_builtin_deps=True we make sure that only
symbols explicitly marked with _PUBLIC_ are exported
and we only link to system libraries and include all
internal depedencies as builtin subsystems.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14780

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>