web-api(feat): include an experimental option for the axios adapter

note: a few steps have to happen to use this and it should be with
caution! these are experimental changes and not stable. For #2212.

this adds clientOptions.experimentalAxiosAdapter to App which accepts
combinations of ['xhr', 'http', 'fetch'] and uses that a the default.

testing setup:

- git clone https://github.com/slackapi/node-slack-sdk
- cd node-slack-sdk
- git checkout web-api-6.x-experimental-axios-adapter
- cd packages/web-api
- npm install
- npm run build
- cd ../../../
- git clone https://github.com/slackapi/bolt-js
- cd bolt-js
- git checkout web-api-6.x-experimental-axios-adapter
- npm install ../node-slack-sdk/packages/web-api
- npm run build
- cd ../path/to/app
- npm install ../path/to/bolt-js
- node app.js

src/App.ts

@@ -104,7 +104,7 @@ export interface AppOptions {
   logger?: Logger;
   logLevel?: LogLevel;
   ignoreSelf?: boolean;
-  clientOptions?: Pick<WebClientOptions, 'slackApiUrl'>;
+  clientOptions?: Pick<WebClientOptions, 'slackApiUrl' | 'experimentalAxiosAdapter'>;
   socketMode?: boolean;
   developerMode?: boolean;
   tokenVerificationEnabled?: boolean;
@@ -363,6 +363,7 @@ export default class App<AppCustomContext extends StringIndexed = StringIndexed>
     this.axios = axios.create({
       httpAgent: agent,
       httpsAgent: agent,
+      adapter: clientOptions?.experimentalAxiosAdapter ?? ['xhr', 'http', 'fetch'],
       // disabling axios' automatic proxy support:
       // axios would read from env vars to configure a proxy automatically, but it doesn't support TLS destinations.
       // for compatibility with https://api.slack.com, and for a larger set of possible proxies (SOCKS or other