Add POST endpoint for additional access methods to existing object

Description

Added a new endpoint to allow posting additional access methods to an existing DRS object.

Fixes #25

New feature

Please delete options that are not relevant.

[x] Bug fix (non-breaking change which fixes an issue)
[x] New feature (non-breaking change which adds functionality)
[x] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[x] Documentation update

[x] My code follows the style guidelines of this project
[x] I have performed a self-review of my own code
[x] I have commented my code, particularly in hard-to-understand areas
[x] My changes generate no new warnings
[x] I have added tests that prove my fix is effective or that my feature works
[x] New and existing unit tests pass locally with my changes
[x] I have not reduced the existing code coverage
[x] I have added docstrings following the Python style guidelines of this project to all new modules, classes, methods and functions are documented with docstrings following; I have updated any previously existing docstrings, if applicable
[x] I have updated any sections of the app's documentation that are affected by the proposed changes, if applicable