* feat: Add connection_timeout and troubleshooting_url to agent
This commit adds the connection timeout and troubleshooting url fields
to coder agents.
If an initial connection cannot be established within connection timeout
seconds, then the agent status will be marked as `"timeout"`.
The troubleshooting URL will be present, if configured in the Terraform
template, it can be presented to the user when the agent state is either
`"timeout"` or `"disconnected"`.
Fixes#4678
This feature is used by the coder agent to exchange a new token. By
protecting the SessionToken via mutex we ensure there are no data races
when accessing it.
* fix: Refactor agent to consume API client
This simplifies a lot of code by creating an interface for
the codersdk client into the agent. It also moves agent
authentication code so instance identity will work between
restarts.
Fixes#3485 and #4082.
* Fix client reconnections
