Follow up discussion from https://phab.comm.dev/D10908?id=36504#inline-66205
- Added `base-64` dependency to lib (it already was in native)
- Extracted `createHTTPAuthorizationHeader` function from D10908 - the header format is described in [[ https://linear.app/comm/issue/ENG-4657/http-services-authorization-middleware | ENG-4657 ]]
- Used `base-64` in the function instead of passing platform-specific b64 implementations