baseOptions for typescript-fetch

[dsumer]

PR checklist

• [X] Read the contribution guidelines.
• [X] Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

add possiblity to pass baseOptions to the API Configuration, which will be passed to every fetch call

useful to pass e.g. { credentials: 'include' } as baseOptions and don't have to pass it to every single API call