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.
