feat(primeng): select filtering prop

[13dante04]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
New feature on PrimeNG select, adds the ability to enable search bar (filter property)
Filtering enabled on the demo page.

What is the current behavior? (You can also link to an open issue here)
Currently if using PrimeNG select with formly, search bar is not available

What is the new behavior (if this is a feature change)?
By adding the
filter: true
property to the props object a basic search bar will now be enabled

Please check if the PR fulfills these requirements

• The commit messages follow our guidelines: https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format
• A unit test has been written for this change.
• Running npm run build produced a successful build. (Unit testing can be done by running npm test;)
• My code has been linted. (npm run lint to do this. npm run build will fail if there are files not linted.)

Please provide a screenshot of this feature before and after your code changes, if applicable.

Other information:

[aitboudad]

Thank you @13dante04!

[aitboudad]

Published in v6.3.1 release.