Add Cache Manager support for Accept header

[yadij]

This completes the support in cachemgr Action classes for incremental addition of report formats other than text/plain (historic cachemgr.cgi accepted syntax).

Reporters (class Action children) can now determine their content-type support dynamically based on request Accept header.

[yadij]

I have dropped all the YAML specific parts. This PR is now just making the Accept header available to report generators.

[rousskov]

I do not understand the current description[^1] of the feature label, but perhaps this PR does not need that mysterious label?

[^1]: maintainer needs documentation update for merge: "For merge" of which PR? Where? It does not sound like we are talking about merging the labeled PR into its target branch... Please update the label name and description to clarify. If this label is about the need to update Squid release notes and such (after the PR is merged), then let's say that explicitly, preferably with a common reusable prefix! For example, todo-release-notes or todo-update-release-docs.