Add CHRONOSPHERE_USER_AGENT env var to override User-Agent header

[codygibb]

The User-Agent header was previously hardcoded
from the component name and build info, with
no way to override it at runtime. This adds a
CHRONOSPHERE_USER_AGENT env var that, when set,
replaces the default User-Agent header value.
Follows the same pattern as CHRONOSPHERE_ENTITY_NAMESPACE
and CHRONOSPHERE_ACTOR.

[codygibb]

This change is part of the following stack:

• Add CHRONOSPHERE_USER_AGENT env var to override User-Agent header #107 ◀

_{Change managed by git-spice.}