Support configuring an already downloaded backend

[matifali]

To make connections fast, a user can download the IDE backend into their workspace image. Can we support specifying the path of the backend to be used by the gateway to establish the connection?

We currently support specifying versions and downlod_url that downloads the given version from the internet if it does not exist on a default location.

Related upstream issue(s):

1. https://youtrack.jetbrains.com/issue/IJPL-170135/Ability-to-add-an-already-installed-IDE-via-Gateway-UI
2. https://youtrack.jetbrains.com/issue/IJPL-166728/API-for-gateway-to-predownload-client
3. https://youtrack.jetbrains.com/issue/IJPL-170129/Background-Automated-Backend-Installation

[code-asher]

You should be able to pass ide_path_on_host to point to an existing backend.

[matifali]

How does it work alongside ide_download_url? Which one takes precedence? Do we have docs?
If not I would like to document this behavior

[code-asher]

I think it is not documented. 🤔 I believe we prefer the path if both are set.

[matifali]

Thank you. I will document this behavior.

[matifali]

While investiagting this. Looks like we already documented this in a form at https://coder.com/docs/user-guides/workspace-access/jetbrains#jetbrains-gateway-in-an-offline-environment

We can repurpose those docs to also serve non offline deployments.

Can you help here @EdwardAngert ?