refactor(http): Simplify Authorizer (#17704)

[ayang64]

Have AuthorizerIsOpen() assert if a given authizer has an
AuthorizeUnrestricted() method and if so, call that to provide the
result of AuthorizerIsOpen().

Otherwise we check if the supplied Authorizer is nil.

This preserves the fast-path for checking tag-level (and other) tsdb
operations.

This simplifies how we handle such authorizers by handling this case in
only one place.

Closes #

Describe your proposed changes here.

• CHANGELOG.md updated with a link to the PR (not the Issue)
• Well-formatted commit messages
• Rebased/mergeable
• Tests pass
• http/swagger.yml updated (if modified Go structs or API)
• Documentation updated or issue created (provide link to issue/pr)
• Signed CLA (if not already signed)