build: Generate TypeScript types from OpenAPI definition

[firstred]

Requires:

• fix: TypeScript detected type issue #58 for the tests to pass

This PR will add a script to automatically generate TypeScript types from bunq's OpenAPI definition.

It looks like not all required properties are marked as such in the OpenAPI definition, so there is a way to add these via the script.
To prevent types from being flattened to just primitives such as string, there is the possibility to extend the definition as well.

Generating a new definition file can be done with yarn build:types. Definitions end up in the file src/Types/ApiTypes.ts

A backward compatible layer has been added.

[codecov]

Codecov Report

Merging #55 into master will decrease coverage by 39.27%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master      #55       +/-   ##
===========================================
- Coverage   98.65%   59.38%   -39.28%     
===========================================
  Files          71       17       -54     
  Lines        2236      517     -1719     
  Branches      461       96      -365     
===========================================
- Hits         2206      307     -1899     
- Misses         29      209      +180     
  Partials        1        1

Impacted Files	Coverage Δ
src/HTTP/SignRequestHandler.ts	6.81% <0%> (-93.19%)	⬇️
src/HTTP/VerifyResponseHandler.ts	10.25% <0%> (-89.75%)	⬇️
src/HTTP/EncryptRequestHandler.ts	17.64% <0%> (-82.36%)	⬇️
src/ApiAdapter.ts	12.12% <0%> (-80.81%)	⬇️
src/Helpers/Utils.ts	77.77% <0%> (-22.23%)	⬇️
src/Crypto/Rsa.ts	81.25% <0%> (-18.75%)	⬇️
src/HTTP/Request.ts	90.16% <0%> (-9.84%)	⬇️
src/Crypto/Aes.ts	94.28% <0%> (-5.72%)	⬇️
src/Api/SchedulePayment.ts
src/Api/Avatar.ts
... and 52 more

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[flow-danny]

Can this be merged? Would very much like to use this 😄