Add header support when using custom request url

[ryanb-jobber]

A more generalized version of #660 that allows arbitrary headers to be specified when using a custom requestUrl. It determines the headers at request time so that values can be specified dynamically.

Our use case is adding an Authorization header to proxied Google requests. The custom proxy allows us to append the Google API key server side so it is not exposed via app traffic (as we can not use a restricted key with this library). We also need to guard against unauthorized usage of the proxy.

Example usage:

requestUrl={{
  url: MY_PROXY,
  useOnPlatform: "all",
  headers: {
    Authorization: `Bearer ${accessToken}`,
   },

[mkilp]

This is desperately needed here too and should be merged soon.

[bell-steven]

This is an excellent contribution. Thanks for the PR!

[bell-steven]

Released as v2.4.0