* Add tracing instrumentation for ssh clients/servers
Add tracing context to the existing ProxyHelloSignature to provide
span information across ssh connections. To add span context per
ssh session on top of new connections, the same tracing context is
passed in the first global request of the session.
In order to ensure that tracing context is pulled from and inserted
into the proper context.Context, some interfaces and methods were
changed to take one as the first argument.
Load access requests from SSH cert instead of the profile. The profile
only exists on CLI clients, but not in the proxy.
Note: theoretically, SSH cert may be missing in some cases for CLI
clients. We should eventually encode access requests in TLS certs too,
which are always present.
* 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.
rosstimothy