Document ProxyJump

[klizhentas]

In SSH terms ProxyJump is a shortcut for SSH client
connecting the proxy/jumphost and requesting .port forwarding to the
target node.

This commit adds support for direct-tcpip request support
in teleport proxy service that is an alias to the existing proxy
subsystem and reuses most of the code.

This commit also adds support to "route to cluster" metadata
encoded in SSH certificate making it possible to have client
SSH certificates to include the metadata that will cause the proxy
to route the client requests to a specific cluster.

tsh ssh -J proxy:port is supported in a limited way:

Only one jump host is supported (-J supports chaining
that teleport does not utilise) and tsh will return with error
in case of two jumphosts: -J a,b will not work.

In case if tsh ssh -J user@proxy is used, it overrides
the SSH proxy coming from the tsh profile and port-forwarding
is used instead of the existing teleport proxy subsystem