hcm: introduce FilterManager (envoyproxy#12295)

Moves all the filter handling into an inner class, in preparation for
splitting the behavior out of the HCM. As an initial PR, this relies on
delegating to the ActiveStream for basically all operations not directly
related to the filters.

This is a relatively minor refactor in that most of the FM logic just
delegates back up to the ActiveStream. In subsequent PRs this logic will
be migrated to the FM piece by piece, until the AS and the FM no longer
are tightly coupled, at which point FM can be moved out into its own file.

Risk Level: Medium
Testing: Existing tests
Docs Changes: n/a
Release Notes: n/a
Part of envoyproxy#10455

Signed-off-by: Snow Pettersen <kpettersen@netflix.com>