All agents should accept additional env vars for flow "jobs"

[cicdw]

Right now, agents set a pre-defined list of environment variables on all flow run jobs which configure Core to run a certain way and talk to Prefect Cloud.

We should open this up a little and allow for users to specify a dictionary of environment variables at agent start time via both __init__ as well as CLI flags.

This would allow for security / sensitive configurations to be set much more seamlessly and securely. E.g., if I want to start a Docker agent which grants S3 access to all of its flow runs, it can set AWS creds on each container job and that doesn't need to be persisted as either a Secret or in code.