feat(webex-core): adding support for a custom proxy

[Tiuipuv]

This pull request addresses

Adding support for an app to set a custom proxy on all requests. The app can control this by setting the proxy configuration during init. e.g.

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

By allowing for custom proxy it'll be easier for organizations utilizing a proxy to be able to connect to the webex sdk via NodeJS.

by making the following changes

These changes include:

• Adding a new interceptor class for webex-core, called ProxyInterceptor
• Adding the interceptor into webex-core, to run against every onRequest
• Adding new unit test file for ProxyInterceptor

Change Type

• [ ] Bug fix (non-breaking change which fixes an issue)
• [X] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation update
• [ ] Tooling change
• [ ] Internal code refactor

The following scenarios where tested

• no proxy given (automated)
• string proxy is given in webex init config (automated)

I certified that

• [X] I have read and followed contributing guidelines
• [ ] I discussed changes with code owners prior to submitting this pull request
• [X] I have not skipped any automated checks
• [X] All existing and new tests passed
• [ ] I have updated the documentation accordingly

---

Make sure to have followed the contributing guidelines before submitting.

[Tiuipuv]

If this PR is accepted, it is recommended that the proxy documentation on this wiki page be updated to include both the websocket proxy setup as well as this new http proxy setup.

[sreenara]

@Tiuipuv, the SDK team will review this PR. Please stand-by.