feat: support migration credentials

[MichaelGoberling]

Description

When a workspace is undergoing credential migration, we get one credential back from the Console API that is of type oauth_server_to_server_migrate, this PR adds support for that case

Related Issue

Motivation and Context

How Has This Been Tested?

• npm run test
• locally linked app plugin with code changes

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #65 (4370e7c) into master (9879c91) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #65   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         5           
  Lines          530       530           
  Branches        90        90           
=========================================
  Hits           530       530           

Impacted Files	Coverage Δ
lib/pure-helpers.js	100.00% <100.00%> (ø)

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