[typescript-fetch] add default config feature

Created by: mkusaka

This PR allows we to write global settings for api clients.

Prior to this, we had to write the following for each api client (tag), which is a bit verbose.

import {
  Configuration,
  ClientAApi,
  ClientBApi,
  ClientCApi,
} from "./generated";

// custom config
const config = new Configuration({
  credentials: "include",
});

export const clientA = new ClientAApi(config);
export const clientB = new ClientBApi(config);
export const clientC = new ClientCApi(config);

After this change, it can be written simply as follows.

import {
  Configuration,
  ClientAApi,
  ClientBApi,
  ClientCApi,
  DefaultConfig,
} from "./generated";

// custom config
DefaultConfig.config = new Configuration({
  credentials: "include",
});

export const clientA = new ClientAApi();
export const clientB = new ClientBApi();
export const clientC = new ClientCApi();

Also, custom configuration for specific client still configurable.

I think this is not breaking change, so I didn't add any new options to this change.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

  Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

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