added/agent: persistent global state for agent

[jamesmcnamara]

Adds a var to ClientCapabilities allowing extensions to specify a file path where the agent can persist global state, such as chat. Currently does the simplest possible thing which is to write the entire state out to disk on every update. This can obviously be improved, but at the cost of complexity.

Test plan

Added unit tests Manually tested this with the Eclipse plugin in https://github.com/sourcegraph/eclipse/pull/39

Changelog

Added ability to persist state from the agent