Basic multi client support

[eligotts]

Description

State['client'] now becomes a dictionary mapping from str -> client object
we then need to add a new field to State to track what the current client is
in init_state, we handle whether the user passed in a dictionary for the llm client mapping or a single client
add a get_next_client function to multi_turn_env to implement client switching logic
decided that get_prompt_messages should own this model switching logic, but not sure if that is best option
for more robust solution, would need prompt message rewrite logic according to what client we are on

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