[ENHANCEMENT] Add `mimir-http-prefix` option to introduce a prefix for Mimir URL when use legacy routes.

[ying-jeanne]

[ENHANCEMENT] Add mimir-http-prefix option to introduce a prefix for Mimir URL when use legacy routes.

What this PR does

clean up some mimirtool bugs, adding mimir-http-prefix for mimir client when use legacy routes

Which issue(s) this PR fixes or relates to

Fixes #8018

Checklist

• [x] Tests updated.
• [x] Documentation added.
• [x] CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• [x] about-versioning.md updated with experimental features.