Add an API to track the lifecycle of GLSP Client connections

[CamilleLetavernier]

Currently, all injected elements in the GLSP Server are relying on garbage collection when they are no longer required. However, some injected members might have to manage some resources (e.g. Threads), and they need to know when to dispose these resources. There is currently no API for this

The Lifecycle API should:

• Be available to any injected member of the GLSP Server module
• Notify when a new clientId is created (i.e. a new GLSP widget for an existing client)
• Notify when a clientId is removed (Widget disposed)
• Notify when a client disconnects (Tab closed)

Note that the initial "client connection" triggers the injection, so we (probably) don't need a specific notification for that.

[tortmayr]

Note that the initial "client connection" triggers the injection, so we (probably) don't need a specific notification for that.

I'm wondering if this LifeCycleAPI should actually be injected. Wouldn't it be better to have a global life cycle manager that can also track all connected clients (actual clients not clientIds). If so, this manager would either have to be singleton or we have to parent injector (one per server).

[CamilleLetavernier]

The Lifecycle itself doesn't have to be created through injection; but it can still be injected through each module.

Even if the Lifecycle is a singleton, we still probably don't need a specific notification for client connection, because injected elements will be created after the client has connected (i.e. the client will connect before any injected instance will have a chance to install a listener on the Lifecycle)