fix(core): fixed FieldArrayTypeConfig props declaration

[dereekb]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Fixes #3341.

What is the current behavior? (You can also link to an open issue here)
The FieldArrayTypeConfig type does not allow for a custom props value like FormlyFieldConfig does.

What is the new behavior (if this is a feature change)?
The FieldArrayTypeConfig type will now allow for a custom props value like FormlyFieldConfig does.

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.)

Other information:

Looks like the types for this interface were overlooked in #3295.

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[aitboudad]

Thank you @dereekb!