Make it easier to run custom agent builds with elastic-package

So, unless I'm completely skipping over something, it's not particularly easy to connect an `elastic-package` stack to an instance of elastic-agent you built yourself. If you run the agent independently of docker, you have to tinker with the global agent settings so the agent has a viable elasticsearch/fleet endpoint, which breaks agents running inside docker.

There also doesn't seem to be a particularly elegant way to connect the docker stack to a custom-built elastic-agent image.

Ideally, there should be some kind of flag that does most of this work:
`elastic-package stack up --build-agent`

Something like this shouldn't be _too_ complicated, as it just needs to:
- Build a new copy of the `elastic-agent` docker container
- spin up an elastic stack orchestrated with that container