Allow and add warning for HTTP anchor-data

[palas]

Changelog

- description: |
    Modified anchor-data checking to allow HTTP schema for testing purposes
  type:
  - feature

Context

Having only support for HTTPS and IPFS is cumbersome, because it would require obtaining an SSL/TLS certificate and a domain for running the tests, or publishing to IPFS with every test (whenever the anchor-data changes).

This PR allows HTTP schema but shows a warning when it is used.

It also ensures we use "scheme" throughout and not "schema" (for self-consistency and consistency with IANA nomenclature), and adds documentations for some functions.

How to trust this PR

There are plenty of tests so, in my opinion, it is mainly about documentation and code structure. I tested the warning manually, let me know if we should add an automated test for it.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff

[Jimbo4350]

LGTM but please remove the errors dependency. You can use hoistMaybe.