[master] rest_cherrypy: accept / force credentials via HTTP header

[Malte-Wagner]

The acceptance of HTTP headers opens the possibility to either set or force credentials for a session.
A reverse proxy, can handle authentication and
inject the custom headers X-Forwarded-User, X-Forwarded-Password and/or X-Forwarded-Eauth. In general, these headers can work in conjunction with the eauth sharedsecret / rest / auto methods to externalize and flexibilize authentication, without raising code complexity for new or niche authentication methods inside salt.

What does this PR do?

This PR enables rest_cherrypy to gather credentials via HTTP headers, that may be set by reverse proxy, to use them for session creation or session checking.

What issues does this PR fix or reference?

#22046

Previous Behavior

Credentials are only allowed via request body.

New Behavior

Credentials are allowed via request body and http header.
Prevalence is http header over request body values.

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

Commits signed with GPG?

Yes

Please review Salt's Contributing Guide for best practices, including the
PR Guidelines.

See GitHub's page on GPG signing for more information about signing commits with GPG.

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