Replace Argo Events stack with Provider Service component

[alexgeorgousis]

Closes #359

Tasks

• Remove old VAI eventing server code (argo/providers/vai/eventing_server.go and argo/providers/vai/provider.go)
• Remove old KFP eventing server code
• Remove EventingServer interface layer along with CLI layer for eventsource-server from argo module
• Integrated tests (possibly with docker-compose spinning up pubsub for VAI and the equivalent for KFP)
• Implement streaming solution for both VAI and KFP to consume from their source and push run completion event data to the HTTP webhook in the operator
• Documentation updated

[alexgeorgousis]

I have a question about the way we've implemented the event processing for providers.

Up until this point, the steps to introduce a new provider (I think) were:

1. Build a publish a docker image that handles CRUD for resources in the new provider and handles events coming out of the new provider
2. Deploy a new Provider resource with that provider image specified

What we've basically done in this PR is move the event processing logic for the existing providers to a separate service.

So as far as I can tell, the steps are now:

1. Build a publish a docker image that handles CRUD for resources in the new provider
2. Deploy a new Provider resource with that provider image specified
3. Raise a PR in the KFP Operator repo to extend the provider-service module with eventing logic for the new provider

Questions:

1. Am I missing something?
2. If not, do we think this is a problem?