[Finder] adds adapter selection/unselection capabilities

As we have many issues with the native finder adapter, it would be good to:
*  permit selection of a particular adapter which will be choosed over the others until its unselection
*  permit unselection of adapters to get the finder select trhe best on (by priority)
*  have `PhpAdapter` selected by default

This PR adds 2 methods to the `Finder`:
*  `setAdapter(AdapterInterface $adapter, $priority = 0)`: selects an adapter (adds it if not alreday registered)
*  `useBestAdapter()`: tells the finder to select best adapter by their priority

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes