feat: added OAuth Server-to-Server Credential support

[shazron]

Requires adobe/aio-lib-console#63

How Has This Been Tested?

• npm test

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 #64 (bba09f6) into master (08d76c4) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #64   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         5           
  Lines          498       530   +32     
  Branches        81        90    +9     
=========================================
+ Hits           498       530   +32     

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

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