feat: Abstract Agent Class + Split Thread Agent

[vivek3141]

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[cpacker]

Abstract agent class could look something like this:

class BaseAgent(object):
    def step(...) -> ...:
       ....

# Then update Agent to inherit from it
class Agent(BaseAgent):
...

# Also the SplitThreadAgent
class SplitThreadAgent(BaseAgent):
...

# And elsewhere in the code we can put in BaseAgent for the typing hint, e.g.:
def save_agent(agent: BaseAgent, ms: MetadataStore):
...