Add context variables to ConversableAgent

[marklysze]

Why are these changes needed?

As a means to support the current SwarmAgent implementation and provide future agent-level context variables, we add context variables to a ConversableAgent. Each agent has their own independent context variables.

This will be a dictionary, Dict[str, Any] and interacting with it should be done through getter/setter functions:

• my_agent.get_context_value - gets a value based on a key (allowing a default to be used)
• my_agent.set_context_value - updates a key with a value
• my_agent.set_context_values - updates with a dictionary
• my_agent.pop_context_key - removes a key from the context variables

E.g.

my_context = {
    "key_1": value_1,
    "key_2", value_2
}

my_agent = ConversableAgent(..., context_variables=my_context)

my_agent.get_context_value("key_1")
my_agent.set_context_value("key_3", something_here)
my_agent.set_context_values({...})
my_agent.pop_context_key("key_1")

Note: If context variables are passed in when establishing a ConversableAgent it must be able to be deep-copied.

docstring for ConversableAgent has been updated - no other specific documentation has been added. Let me know if it needs it at this stage.

Related issue number

PR #104 is a dependency on this.

Checks

• I've included any doc changes needed for https://ag2ai.github.io/ag2/. See https://ag2ai.github.io/ag2/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[sonichi]

LGTM with one optional suggestion.