[typescript-fetch] add bearer token support

[ken210]

PR checklist

• Read the contribution guidelines.
• 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, ./bin/openapi3/{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\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
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Description of the PR

First of all thanks for such great project and for taking your time to review this PR. I have never contributed to this project before, so I may be missing something here.

This PR aims to add support for Bearer authentication token to typescript-fetch generator.
This PR is inspired by: mrvdot@82f409c

Important notes:

I had to change the type signature for
Configuration.accessToken and Configuration.apiKey. This change apparently didn't break anything but I can't be completely sure.