Add support for PMPro object fields

[sc0ttkclark]

Is your feature request related to a problem? Please describe.
Add support for PMPro object fields based on each context.

• PMPro Users would have access to membership level(s), etc
• PMPro Orders would have access to fields on the order
• PMPro Membership Levels would have access to fields on the level

With added support for recognizing/mapping object fields on these three Pods, users can then use Pods templating / magic tags to better interact/render those values using the power of Pods relationships/traversal/display formatting etc.

Describe the solution you'd like
Filter the WP object fields in Pods for the extended PMPro objects. Add fields in such a way that it recognizes their labels, formats, and display options.

Describe alternatives you've considered
The code currently relies on the data returned from the object's table but it does not recognize formatting and it just outputs the value as plain text. It also does not auto-suggest those fields when using the Pods template builder.

Requested by: https://www.paidmembershipspro.com/forums/topic/show-membership_status-with-pods-template/