Full Proxy Support

[Tiuipuv]

Is your feature request related to a problem? Please describe.
Currently, certain corporate environments cannot start or use the webex sdk due to proxy requirements. The current version of the SDK supports setting a proxy for the mercury plugin, but does not include support for generic https fetch requests. This feature would allow these restrictions to be lifted, as the user would be able to specify the proxy in the setup of the sdk.

Describe the solution you'd like
Developed in PR 3624, The user would be able to specify a proxy string on initialization, as shown below:

webex = WebexSdk.init({
  credentials: {
    supertoken: superToken
  },
  config: {
    credentials: {
      client_id,
      client_secret
    },
    proxy: 'http://proxy.company.com'
  }
});

Describe alternatives you've considered
No overrides or alternatives are available, as the request library under the hood does not have any other exposure points for configuration.

Additional context
Add any other context or screenshots about the feature request here.

[Tiuipuv]

Can the following branch be linked for issue resolution by a contributor/admin of the sdk?

#3624