feat(primeng): added multiselect

[13dante04]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
This PR adds the multiselect control from primeng to formly, it also adds it to the demo for primeng.

What is the current behavior? (You can also link to an open issue here)
There is no multiselect component from primeng.

What is the new behavior (if this is a feature change)?
PrimeNG's multiselect can now be used with formly

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.

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