feat: initialize openfunction knative and async runtime

[webup]

This big initial PR is an important upgrade to current GCP Node.js Function Framework with following task accomplished:

• Add OpenFunction knative runtime
• Add OpenFunction async runtime
• Add e2e tests for knative runtime
• Add e2e tests for async runtime
• Add conformance test for knative runtime
• Add conformance test for async runtime
• Add sequence diagram for knative runtime
• Add sequence diagram for async runtime
• Test runtimes in Kubernetes
• Update auto-generated API docs
• Refine README

[benjaminhuo]

Big kudos to this PR, just some comments for some corner cases

[webup]

Is it reasonable to make adjustment like below to be consistent with go functions framework?

send(data: object, output?: string)
or
send(output?: string, data: object) ?

In TS, a required param cannot be placed after an optional param.

[benjaminhuo]

Is it reasonable to make adjustment like below to be consistent with go functions framework?

send(data: object, output?: string) or send(output?: string, data: object) ?

In TS, a required param cannot be placed after an optional param.

If the output is not set, where is the data sent to?

[webup]

Is it reasonable to make adjustment like below to be consistent with go functions framework?

send(data: object, output?: string) or send(output?: string, data: object) ?

In TS, a required param cannot be placed after an optional param.

If the output is not set, where is the data sent to?

Will broadcast to all declared outputs In current impl.

[benjaminhuo]

The broadcast could be a different method, the send just sends the output to one destination like go implementation maybe?

[webup]

The broadcast could be a different method, the send just sends the output to one destination like go implementation maybe?

As we discussed in wechat group, let's move on to roll out 0.4.0 first, then will upgrade the function interfaces to better comply with the Function Framework overall guidelines. :)

[benjaminhuo]

send(data: object, output?: string) seems to be language-specific, there are optional params in nodejs while there isn't in go. We can keep it for now.