This helper returns the token as well as the ACL policy, to be used in a later
commit for logging the token info associated with nomad exec invocation.

Test helper that allows registration of jobs when ACL is activated.

Extract command parsing and execution mocking into a separate struct.  Also,
allow mocking of different fs_isolation for testing.

This adds `alloc-exec` capability to allow operator to execute command into a
running task.  Furthermore, it adds `alloc-node-exec` capability, required when
the alloc task is raw_exec or a driver with no FSIsolation.

In this commit, we add two driver interfaces for supporting `nomad exec`
invocation:

* A high level `ExecTaskStreamingDriver`, that operates on io reader/writers.
  Drivers should prefer using this interface
* A low level `ExecTaskStreamingRawDriver` that operates on the raw stream of
  input structs; useful when a driver delegates handling to driver backend (e.g.
  across RPC/grpc).

The interfaces are optional for a driver, as `nomad exec` support is opt-in.
Existing drivers continue to compile without exec support, until their
maintainer add such support.

Furthermore, we create protobuf structures to represent exec stream entities:
`ExecTaskStreamingRequest` and `ExecTaskStreamingResponse`.  We aim to reuse the
protobuf generated code as much as possible, without translation to avoid
conversion overhead.

`ExecTaskStream` abstract fetching and sending stream entities.  It's influenced
by the grpc bi-directional stream interface, to avoid needing any adapter.  I
considered using channels, but the asynchronisity and concurrency makes buffer
reuse too complicated, which would put more pressure on GC and slows exec operation.

Also add a helper that converts the adapts the high level interface to the
low-level interface of nomad exec interfaces.

Add a client streaming RPC endpoint for processing nomad exec tasks, by invoking
the relevant task handler for execution.

This adds a websocket endpoint for handling `nomad exec`.

The endpoint is a websocket interface, as we require a bi-directional
streaming (to handle both input and output), which is not very appropriate for
plain HTTP 1.0. Using websocket makes implementing the web ui a bit simpler. I
considered using golang http hijack capability to treat http request as a plain
connection, but the web interface would be too complicated potentially.

Furthermore, the API endpoint operates against the raw core nomad exec streaming
datastructures, defined in protobuf, with json serializer.  Our APIs use json
interfaces in general, and protobuf generates json friendly golang structs.
Reusing the structs here simplify interface and reduce conversion overhead.

Add consolidated testing package to serve as conformance tests for all drivers.

Adds nomad exec support in our API, by hitting the websocket endpoint.

We introduce API structs that correspond to the drivers streaming exec structs.

For creating the websocket connection, we reuse the transport setting from api
http client.