Add a method to forward a specific port to local app api

[atduarte]

When opening a workspace with Jetbrains IDEs, we will need to establish an SSH connection and forward the IDE backend port. Currently, the only options would be to:

1. Use the local app to auto-tunnel all ports of the workspace. Not ideal as the IDE will also port forward after the connection is established showing random ports to the users.
2. Implement port forwarding in the local IDE plugin. Not ideal as we would duplicate the logic and split the responsibility of establishing and managing the connection.

[atduarte]

We have another related issue. As the IDE backend port will be internal, the local app is incapable of tunneling it (see internal discussion).

The plan is to proceed with #5642 first.

/cc @akosyakov

[akosyakov]

We need to verify it yet, but it seems that Gateway provides APIs capable to forward ports including of a tcp join link. We should use the local companion to establish a SSH tunnel to a workspace and then provide Gateway with SSH credentials and a tcp join link. Gateway should be able to do the rest. I put the design here: https://www.notion.so/gitpod/JetBrains-server-to-client-flow-09a1262189054a21bcf7761ab0c54a64#5c5e814e4d68481d94d3dddd4f00fe68

[akosyakov]

Confirmed, managed to build a prototype with manually started local companion and tpc join link.

[akosyakov]

We don't use lcoal app for desktop IDEs integrations anymore but SSH gateway.