Workspace Agent Prebuild Awareness and Authentication

[SasSwart]

When a prebuilt workspace is claimed and transferred to a user, minor updates to the workspace need to be made to adapt to the new owner. To facilitate a subset of these updates, workspace agents inside the former prebuild need to reinitialise themselves. Updates need to be introduced to the agent cli, the agent itself and the agentsdk to listen for instructions to reinitialise and action them. The workspaceagents.go API should also have a new endpoint to produce server sent events that instruct workspace agents to reinitialise.

Depends on: coder/terraform-provider-coder#364