Adding oneOf with discriminator property support

[sjoerdmulder]

Continuation of #645 but in a cleaned branch...

Currently it seems there is no support for discriminators from the V3 spec in the generator.

This PR adds some preliminary support so that instead of:

export type ModelCircle = {
    kind: string;
    radius?: number;
}
export type ModelSquare = {
    kind: string;
    sideLength?: number;
}
export type Shape = Circle | Square;

It generates (not the the kind: string is replaced)

export type ModelCircle = {
    kind: 'circle';
    radius?: number;
}
export type ModelSquare = {
    kind: 'square';
    sideLength?: number;
}
export type Shape = Circle | Square;

Added some tests as well, let me know what you think!

[codecov]

Codecov Report

Merging #741 (f69bdf0) into master (1508588) will increase coverage by 3.16%.
The diff coverage is 96.29%.

❗ Current head f69bdf0 differs from pull request most recent head d82f3eb. Consider uploading reports for the commit d82f3eb to get more accurate results

@@            Coverage Diff             @@
##           master     #741      +/-   ##
==========================================
+ Coverage   87.49%   90.65%   +3.16%     
==========================================
  Files         109      109              
  Lines        1599     1552      -47     
  Branches      311      304       -7     
==========================================
+ Hits         1399     1407       +8     
+ Misses        200      145      -55     

Impacted Files	Coverage Δ
src/utils/discriminator.ts	95.00% <95.00%> (ø)
src/openApi/v3/parser/getModel.ts	100.00% <100.00%> (+0.84%)	⬆️
src/openApi/v3/parser/getModelProperties.ts	100.00% <100.00%> (ø)
src/openApi/v3/parser/getModelComposition.ts	76.19% <0.00%> (-20.69%)	⬇️
src/utils/getOpenApiSpec.ts	81.25% <0.00%> (-18.75%)	⬇️
src/openApi/v3/parser/getContent.ts	58.82% <0.00%> (-10.41%)	⬇️
src/utils/writeClientCore.ts	61.53% <0.00%> (-2.75%)	⬇️
src/index.ts	80.00% <0.00%> (-0.77%)	⬇️
src/openApi/v2/parser/getModel.ts	98.88% <0.00%> (-0.05%)	⬇️
src/utils/writeClient.ts	96.29% <0.00%> (ø)
... and 46 more

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[aperkaz]

@sjoerdmulder @ferdikoomen I am looking forward to this update, is there anything that can be done to get it down to develop?

[sjoerdmulder]

@ferdikoomen I have updated the branch to be based on latest master