Allows to control diagnostics push to the LSP client on behalf of the
LSP server background processes. The pushDiagnostics is a server-initiated
message. Traditionally the diagnostics are collected and pushed whenever a
file is operated on, but there may be different events (for example a background
analytical process finishes) that may change the diagnostic information for a file.
Returns a Control object appropriate for the context and the file. The returned
object can be used to fire changes to LSP client(s). It is important to call
the method while the context is in effect, i.e. during the client's request. May
return null if no suitable LSP can be found.
context - the Optional. Context used to identify the LSP client. If null,
the default Lookup will be used.
file - Optional. The file or folder whose diagnostic will be reported.