* SSH request tracing
Add tracing support for ssh global requests and channels. Wrappers
for `ssh.Client`, `ssh.Channel`, and `ssh.NewChannel` provide a
mechanism for tracing context to be propagated via a `context.Context`.
In order to maintain backwards compatibility the ssh.Client wrapper
tries to open a TracingChannel when constructed. Any servers that
don't support tracing will reject the unknown channel. The client
will only provide tracing context to servers which do NOT reject
the TracingChannel request.
In order to include pass tracing context along all ssh payloads
are wrapped in an Envelope that includes the original payload
AND any tracing context. Servers now try to unmarshal all payloads
into said Envelope when processing messages. If there is an Envelope
provided, a new span will be created and the original payload will
be pass along to handlers.
Part of #12241
* Added support for connecting API client through tunnel proxy and web proxy addresses (with identity file).
* Added concurrent dialing logic to dial several possible dialing combinations and seamlessly return the first client to connect.
Jakub Nyckowski