feat: provide method to set request headers

[aarongranick-okta]

Adds a new sub-api, "http" with one method (so far), setRequestHeader. This allows setting the "headers" option after the AuthJS has already been constructed. Options set here will effect both IDX and non-IDX requests.

The primary purpose of this commit is to set custom request headers on IDX requests, specifically for device fingerprinting. This is needed to unblock removing idx-js dependency from the widget.

One (nice) side-effect of this commit is that the full "sdk" user-agent will now be sent on IDX API requests.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 95.91837% with 2 lines in your changes missing coverage. Please review.

Project coverage is 91.58%. Comparing base (d1aa910) to head (e353d0b).
Report is 416 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/oidc/endpoints/token.ts	66.66%	1 Missing ⚠️
lib/tx/api.ts	75.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #891      +/-   ##
==========================================
+ Coverage   91.54%   91.58%   +0.04%     
==========================================
  Files         117      120       +3     
  Lines        3359     3375      +16     
  Branches      694      695       +1     
==========================================
+ Hits         3075     3091      +16     
  Misses        284      284              

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[shuowu]

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