Release local agent as part of Prefect Core

[jlowin]

Rather than a separate repo, the local Agent should ship with Prefect Core, in order to minimize frictions to getting started with Prefect Cloud.

For a user who had already set an auth token, connecting to the Prefect Cloud platform could be as easy as:

prefect agent start

(or something)

[joshmeek]

So since the agent will be pip installable I think the agent code shouldn’t live in core but become an optional dependency which this function will use

[jlowin]

It's a good point @joshmeek, I think it could be advantageous to have the shared Agent code in its own repo. I don't have a strong opinion there. The main gist of this issue is that we should ship that code with Core in an easily accessible way, so that every single user of the open source can instantly and seamlessly run their code on the Cloud platform.

[joshmeek]

As per offline discussion w/ @cicdw and @jlowin the code will be moved into core under src/prefect/agent for all platforms