[lib] Add function to create HTTP Authorization header

Summary:
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 ENG-4657
• Used base-64 in the function instead of passing platform-specific b64 implementations

Test Plan:

• Repeated test plan from D10908 (also repeated test plans for web and native from future diffs to make sure it works on all platforms)
• Added unit test

Reviewers: ashoat, tomek, michal

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D10929