Weighting: Add ep_enable_do_weighting to check for request type and re-factor with apply_weighting() into do_weighting()

[rebeccahum]

Description of the Change

This PR does two things:

1. Adds the filter ep_enable_do_weighting, which combines the logic of checking for an integrated request (that way one can customize it to their liking as well, if they wanted to change the integrated request types allowed)
2. Re-factors the logic to apply weighting into its own function apply_weighting()

Alternate Designs

For point 2, it would be possible to just filter out the request types itself, but not sure if that's worth adding yet another filter on top of things already.

Possible Drawbacks

N/A

Verification Process

1. Set up EP with search weighting added by UI (pick whatever values)
2. Verify post weighting is still being added as expected on a front-end search for "bar"
3. Add filter to only disable post weighting for searches of "foo":

// Enable weighting only for searches of "foo"
// Enable weighting only for searches of "foo"
add_filter( 'ep_enable_do_weighting', function( $enable_do_weighting, $weight_config, $args, $formatted_arg ) {
   $enable_do_weighting = ElasticPress\Utils\is_integrated_request( 'weighting', [ 'public', 'rest' ] ) && $args['s'] === 'foo';

   return $enable_do_weighting;
}, 10, 4 );

4. Verify post weighting is no longer being added on a search query for "foo"

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Changelog Entry

Added: Adds ep_enable_do_weighting filter and re-factors with new function apply_weighting

Credits

Props @felipeelia @rebeccahum