Add container source plugin

[yan12125]

Closes #59

Uploaded as a draft to see if CI passes

This PR introduces a new dependency for decoding one of OAuth2 header. If you don't want an additional dependency, please tell me and I will try to implement the decoding. (should be less than 100 lines, hopefully) Copied needed parts from Werkzeug

Tested images:

• docker.io/library/python
• docker.io/library/archlinux (for testing caching of auth info)
• registry.gitlab.com/yan12125/yan12125.gitlab.io/website
• quay.io/prometheus/node-exporter
• gcr.io/google.com/cloudsdktool/cloud-sdk
• ghcr.io/br3ndonland/inboard

[yan12125]

It's overkill to depend on werkzeug to just use one small function (and the implementation is incomplete). We can implement this ourselves.

No problem. I copied and simplified needed part from Werkzeug (BSD license).

Also we can cache the auth_info in case a configuration file has a few entries for the same registry.

Good suggestion! Done in the latest revision.

[lilydjwg]

Thanks!