cachekey: added --canonical-prefix parameter

[gtenev]

In certain use-cases when calculating the prefix (the initial value
of the new cache key) we need to have the scheme, host and port in
their original form from the request URI, i.e. when hosting.config
is used the cache key is expected to contain a valid URI authority
element used for volume selection.

More details about the new parameter and its functionality can be
found in doc/admin-guide/plugins/cachekey.en.rst

[gtenev]

This patch fixes a problem found by @knutsel while using cachekey plugin and hosting.config where volume selection failed since the cachekey by design always generates a cache key in a path-like format and removes the URI scheme and authority elements in their original form (which are expected in this case).

I would like to have this PR back ported to 9.x as a "bug fix" for this broken use-case which would allow cachekey plugin to be used with hosting.config without cumbersome workarounds in the cachekey configs (i.e. like using cacheurl compatibility mode and custom delimiters, see cachekey plugin documentation for more details)

[gtenev]

An offline chat with @ezelkow1 about his experiments with this new feature made me realize that an arbitrary combining of all prefix related parameters could also yield a valid host name in the final resulting cache key, depending on the static value provided (--static-prefix) or the success of the regex capture/replace operations (--capture-prefix and --capture-prefix-uri) requested by the config. Added a tweak to cover those cases.

[bryancall]

[approve ci autest]

[ezelkow1]

Looks good, tested here with various scenarios. After the last fix push it seems to work as advertised

[zwoop]

Cherry-picked to v9.0.x branch.

[zwoop]

Cherry-picked to 8.1.x