Extension.get API - Only apply special filters once (key/keys/full_name)

Extension.get API - Only apply special filters once (key/keys/full_name)

Filtering extensions by key is a good idea -- so good we have three ways to do it! ;)

All of these fields accept a `string|array<string>` input, which is different from
the typical filtering supported by `_civicrm_api3_basic_array_get()` (which only
matches on values)...  If you happen to choose a basic filter that passes both
filters (i.e.  a simple string-match), then it works.  But if you choose a filter
where they behave differnetly (i.e.  an array), then you get weird results.