This is the OSS portion of enterprise PR 2339.
It improves our handling of "irrecoverable" errors in proxycfg data sources.
The canonical example of this is what happens when the ACL token presented by
Envoy is deleted/revoked. Previously, the stream would get "stuck" until the
xDS server re-checked the token (after 5 minutes) and terminated the stream.
Materializers would also sit burning resources retrying something that could
never succeed.
Now, it is possible for data sources to mark errors as "terminal" which causes
the xDS stream to be closed immediately. Similarly, the submatview.Store will
evict materializers when it observes they have encountered such an error.
This is the OSS portion of enterprise PR 2141.
This commit provides a server-local implementation of the `proxycfg.Intentions`
interface that sources data from streaming events.
It adds events for the `service-intentions` config entry type, and then consumes
event streams (via materialized views) for the service's explicit intentions and
any applicable wildcard intentions, merging them into a single list of intentions.
An alternative approach I considered was to consume _all_ intention events (via
`SubjectWildcard`) and filter out the irrelevant ones. This would admittedly
remove some complexity in the `agent/proxycfg-glue` package but at the expense
of considerable overhead from waking potentially many thousands of connect
proxies every time any intention is updated.
