[ENG-4961] Allow s3 full configuration via V2 API

[Johnetordoff]

Purpose

S3 should now support “Write-only” fields for s3 specific credentials. This will allow users to configure S3 addons fully through the v2 API.

Changes

• adds new custom serializer that allows s3 authorization.
• adds tests

QA Notes

Expected payloads for:
Credentials

{ 'data': {'attributes': {'access_token': 'test_access_key', 'secret_token': 'test_secret_key'}}}

Folders

{'data': {'attributes': {'folder_id': 'test_folder_id', 'folder_path': 'test_folder_path:/'}}}

For "pre-configuration" info, like bucket and subfolder names use the /v2/nodes/{node._id}/addons/s3/folders/ endpoint

Documentation

Information about setting the site up locally can be found here at addons/s3/README.md.

Side Effects

Ticket

https://openscience.atlassian.net/browse/ENG-4961