commit to illustrate changes for custom-http-client (do not merge)

[nils4cosee]

This example illustrates the feature-request in #400

The library-user can specify a custom function that sends requests to the backend (as illustrated in test/e2e/v3.custom.spec.js

Helper functions like "getToken" and "getUrl" are not provided and must be written by the library-user.

The downside is that ApiResult and ApiRequestOptions would become part of the official API, which will impose restrictions in SemVer-versioning. I can understand if you don't want that.

Final polishing and tests are still missing. Before I go any further I would like to hear your opinion.

[codecov]

Codecov Report

Merging #407 (aacefff) into master (203b74e) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #407      +/-   ##
==========================================
- Coverage   93.39%   93.37%   -0.03%     
==========================================
  Files         108      108              
  Lines        1272     1267       -5     
  Branches      229      229              
==========================================
- Hits         1188     1183       -5     
  Misses         84       84              

Impacted Files	Coverage Δ
src/index.ts	77.27% <ø> (ø)
src/openApi/v2/parser/getMappedType.ts	100.00% <100.00%> (ø)
src/openApi/v2/parser/getType.ts	91.66% <100.00%> (+0.75%)	⬆️
src/openApi/v2/parser/stripNamespace.ts	100.00% <100.00%> (ø)
src/openApi/v3/parser/getMappedType.ts	100.00% <100.00%> (ø)
src/openApi/v3/parser/getType.ts	91.66% <100.00%> (+0.75%)	⬆️
src/openApi/v3/parser/stripNamespace.ts	100.00% <100.00%> (ø)
src/utils/registerHandlebarTemplates.ts	100.00% <100.00%> (ø)

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[ferdikoomen]

Closing this PR in favor of #434 and already existing option to overwrite request.ts