feat: User agreements API for generic agreement records

[xitij2000]

Description

This change adds a new kind of generic user agreement that allows plugins or even the core platform to record a user's acknowledgement of an agreement.

Supporting information

NA

Testing instructions

• This creates a new REST API at /api/agreements/v1/agreement/<agreement_id>
• You can use GET, which will return a 404 if a user hasn't acknowledged the agreement, or the timestamp of acceptance if they have
• You can do a simple empty POST to mark acknowledgement
• You can use the ?after=... query param with get to filter agreements after that date

Deadline

"None"

Open Questions

• Perhaps the text of the licence / agreement etc should also be stored in a django model and versions in an update-only manner.

[openedx-webhooks]

Thanks for the pull request, @xitij2000!

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