Source restructure

[BruceMacD]

Closes: #519

Iteration for: #435

Next steps:
#518 - rename source to providers
#244 - adding the config migrator at the same time as the provider change to reduce the amount of config versions
#435 - add the provider CRUD endpoints

Changes:
Sources now have this structure:

{
    "clientID": "1234",
    "clientSecret": "infra-registry-okta/clientSecret",
    "created": 1635279117,
    "domain": "dev-0.okta.com",
    "id": "abc",
    "kind": "okta",
    "okta": {
      "apiToken": "infra-registry-okta/apiToken"
    },
    "updated": 1635279397
  }

This will allow for a repeatable structure in the future as we only support the auth code flow at this time. For example another custom IDP called some-idp may look like this:

{
    "clientID": "1234",
    "clientSecret": "some-idp/clientSecret",
    "created": 1635279117,
    "domain": "dev-0.some-idp.com",
    "id": "abc",
    "kind": "some-idp",
    "some-idp": {
      "apiToken": "some-idp/apiToken",
      "enableRegistration": true
    },
    "updated": 1635279397
  }

[jmorganca]

LGTM, after revisiting @mxyng 's comments on serialize and deserialize