feat: validation caching and allow list

[MichaelGoberling]

Description

This PR adds the validation caching functionality currently used by the shared validator actions.

It also adds a new function that allows implementers to validate a token against an allow-list of IMS clients.

This should not be breaking as the validateToken call should return the same data as before. The old validateToken function has been moved to a private function _validateToken, and has been slightly modified to work with ValidationCache.

Motivation and Context

To use in the App Builder Delete Service and allow others to cache validations / invalidations. To allow validating a token against an allow-list.

How Has This Been Tested?

Ran npm run test, branch tested against Delete Service ims-auth PR

Screenshots (if appropriate):

Types of changes

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

Checklist:

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

[codecov[bot]]

Codecov Report

Merging #112 (e3ec3b8) into master (b11c0d8) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #112   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            8         9    +1     
  Lines          428       481   +53     
  Branches        58        68   +10     
=========================================
+ Hits           428       481   +53     

Impacted Files	Coverage Δ
src/ValidationCache.js	100.00% <100.00%> (ø)
src/ims.js	100.00% <100.00%> (ø)
src/index.js	100.00% <100.00%> (ø)

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