Watch: Refactor GraphQL subscription handling

[ansgarm]

Currently the subscription handling comes with a few drawbacks which we should address

terraform-cdk/packages/cdktf-cli/lib/server/server.ts

Lines 52 to 54 in 962ffa7

	// subscribe returns an unsubscribe function but that connect reliably
	// be used with PubSub – that lib is recommended by Apollo docs but seems to
	// be unmaintained, maybe ditch it? see https://github.com/apollographql/graphql-subscriptions/issues/240#issuecomment-767568596

terraform-cdk/packages/cdktf-cli/lib/server/server.ts

Lines 73 to 75 in 962ffa7

	// we should get rid if this hack somehow.
	// this has to be done because the client won't receive this
	// error if it is published before the iterator has been returned

Background

The initial implementation of the new cdktf watch command also contains an
experiment in regards to separating the underlying invocation of a synth and of
Terraform for init, plan and apply from the presentation layer (which is in our
case a CLI build with React components using ink). The other commands (e.g.
cdktf deploy) which we currently have, make use of React Hooks for
orchestrating the underlying steps that are required. Quite early it turned out
to be unreasonable to implement the dynamic nature of the watch command using
those same tools. Instead we opted for trying out a separate process and using a
GraphQL api to connect the two. The main reason for this (as opposed to e.g.
ProtoBuf or REST) was the superior tooling support that exists for this in
TypeScript and React. However, with this prototype now in the open, we will
carefully examine it and assess the pros and cons before jumping ship.

(from #817)

[ansgarm]

#1658 removed GraphQL, so this is obsolete 🎉

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