Libraries registry provider for asab.library

[ateska]

This MR adds another library provider for asab.library: Libraries registry provider.
This provider enables a smooth content distribution (technically over HTTPS) and provides an alternative for Git provider.
The content is prepared (ie by CI/CD) as an .tar.xz archive on the HTTPS static web site (such as Azure storage) and this provider periodically downloads it and unpack locally - similarly to git.
It also provides caching, so if the content has not been changed, nothing is downloaded.

It implements the fallback mechanism: multiple servers can be specified in the URI so that the content can be distributed from multiple location, providing higher resiliency.

The URL & configuration form:

[library]
providers=
  ...
  libsreg+https://server1.example.com,server2.example.com/my-library#version
  ...

If version is not provided, then master is used.

[ateska]

Review

[ateska]

See my comment below.

[ateska]

I added TODOs into - slightly refactored - code during the review process.
@mithunbharadwaj - please implement these TODOs.