Rather than defining a distinct `Parameter` class for Gecko, this now uses the
`extend_parameters_schema` utility function (which mobile repos are already
using).

As a consequence, shared parameters are now defined in standalone taskgraph.
And only Gecko-specific parameters are listed in
`gecko_taskgraph/parameters.py`

The only exception is `project` which gets redefined so we can override the
standalone taskgraph default (since it derives `project` from the repo name,
which doesn't work for Gecko).

Differential Revision: https://phabricator.services.mozilla.com/D134515