This bumps protoreflect to v1.15.1, which should correct some
long-running issues WRT downstream dependencies and protoreflect's use
of grpc_testing, unblocking downstream dependencies from upgrading the
grpc package again.

As far as I know, there is no method `NewManagedClient`, but managed clients are created using `NewClient` with `ClientConfig.Managed = true`

#270)

* Adds a new set of interfaces in a `runner` package to allow custom implementations for running plugins.
* Adds 3 new client-facing config options:
  * `RunnerFunc` can be supplied to customise how a plugin is run, with `*exec.Cmd` treated as the spec.
  * `SkipHostEnv` tells go-plugin not to add the host process' environment variables to the command spec
  * `ReattachConfig.ReattachFunc` can be supplied to hook into running plugins that are identified by something other than a process ID
* From a server (plugin) point of view; Adds optional environment variables to control the plugin's listener unix sockets
* Adds `CmdRunner` as the default `Runner` implementation, maintaining existing functionality and backwards compatibility

A set of small follow-ups to #270:

* Make `ClientConfig.Cmd` and `ClientConfig.RunnerFunc` mutually exclusive
* Clients setting `RunnerFunc` can call `ReattachConfig()` but need to supply their own `ReattachFunc` as it can't necessarily be derived from `RunnerFunc`. Exercise `ReattachConfig()` in tests in a way that previously panicked.
* Improve 1 logger line for non-cmd implementations.
* Add `ID()` function to client; accommodates creating a `ReattachFunc` and also useful for client's debug log information.

* Allows us to properly respect the start timeout for implementations that do long-running
  work in the Start method.
* Adding context.Context to Wait and Kill makes them more consistent with Start, but is
  also a bit of a hedge, allowing room in the interface for future features without
  having to break the API.

* Interface change but non-breaking as `runner.Runner` has not yet been tagged
* Differentiate between empty line and closed channel
* Maintain the previous first line of the error message just in case of consumers depending on it

Previously, we only supported setting the group for the server-side
socket. This change makes it possible to set it on the client side as
well. Also fixes a bug where the gRPC broker on the server side would
previously not consume the directory/group environment variables.

* Add TempDir option to UnixSocketConfig. Allows clients to specify
  a folder where plugin-specific Unix socket directories should be
  created. Still defaults to $TMPDIR (if set) or /tmp.
* Improve UnixSocketConfig field names and comments
* Document exported Unix socket environment variables