registration: Let extensions add PHP extension requirements

registration: Let extensions add PHP extension requirements

This change adds the possibility to specify ext-* keys under the 'platform'
key introduced in I6744cc0be2 to require given PHP extensions. Note that
it's impossible to add constraints different from '*', as there is no universal
way to retrieve PHP extension versions.

Bug: T197535
Change-Id: I510de1e6d80f5c1d92dc1d1665aaa6c25bf28bf7