[#416] New endpoint to get credentials via Ajax calls.

[minnur]

This one is related to this apigee/apigee-edge-drupal#434

because show/hide credentials functionality altered in the kickstarter I had to make the change here as well.

[minnur]

Important: This should not be merged before this apigee/apigee-edge-drupal#434

[arlina-espinoza]

@minnur Please see the comment about changes requested in apigee/apigee-edge-drupal#434 (review) to do access checking on the credentials endpoint. If implemented, this PR needs to be updated too.

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[arlina-espinoza]

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[arlina-espinoza]

Added bumping the required apigee_edge version to 8.x-1.13 because this should be merged and released after apigee/apigee-edge-drupal#457 is released (on 8.x-1.13).

[arlina-espinoza]

LGTM, but holding off merging until apigee_edge 8.x-1.13 is released.