Previously, we'd begin a session with the xDS concurrency limiter
regardless of whether the proxy was registered in the catalog or in
the server's local agent state.
This caused problems for users who run `consul connect envoy` directly
against a server rather than a client agent, as the server's locally
registered proxies wouldn't be included in the limiter's capacity.
Now, the `ConfigSource` is responsible for beginning the session and we
only do so for services in the catalog.
Fixes: https://github.com/hashicorp/consul/issues/15753
Introduces two new public gRPC endpoints (`Login` and `Logout`) and
includes refactoring of the equivalent net/rpc endpoints to enable the
majority of logic to be reused (i.e. by extracting the `Binder` and
`TokenWriter` types).
This contains the OSS portions of the following enterprise commits:
- 75fcdbfcfa6af21d7128cb2544829ead0b1df603
- bce14b714151af74a7f0110843d640204082630a
- cc508b70fbf58eda144d9af3d71bd0f483985893
Dan Upton