Add remote folder picker to file sync GUI

[spikecurtis]

Add a remote folder picker to the file sync GUI. This makes it so that you can visually browse the file system on the remote workspace, instead of needing to open a terminal to figure out the path (or type it from memory).

Wireframe below, but we don't need to implement the local browser for this ticket.

Browsing the remote system will be via the 'ls' API we add to the agent HTTP API. The Coder Desktop client can make an HTTP request to the active agent over Coder VPN.

Dependencies

• Add basic file sync support using GUI with manually entered paths #63
• Implement file 'ls' API on Agent internal#382