[API] Add manually post grades setting to assignments API

[yehuda_davis]

Problem statement:

The API currently does not allow the read-only Assignment parameter "post_manually" to be changed via a PUT request to create or edit an assignment. Use case: creating assignments that have a different grade-posting rule than the gradebook/course default

Proposed solution:

Allow "post_manually" to be changed via a PUT request to the Assignments endpoint so that the grade-posting rule can be set for each individual assignment. This was already implemented at the course level by user request here: https://community.canvaslms.com/t5/Canvas-Ideas/API-Add-manually-post-grades-setting-to-course-API/i... Thank you!

User role(s):

instructor

[yehuda_davis]

This was also requested here: https://community.canvaslms.com/t5/Canvas-Developers-Group/Can-Assignment-s-quot-post-manually-quot-...