feat: add Cohere API support (Command-R+)

[cpacker]

Example usage:

% memgpt configure
Loading config from /Users/loaner/.memgpt/config
? Select LLM inference provider: cohere
? Enter your Cohere API key (see https://dashboard.cohere.com/api-keys): ****************************************
? Override default endpoint: https://api.cohere.ai/v1
? Select default model: command-r-plus
Got context window 116000 for model command-r-plus
? Select your model's context window (see https://docs.cohere.com/docs/command-r): 116000
? Select embedding provider: openai
? Select default preset: memgpt_chat
? Select default persona: sam_pov
? Select default human: basic
? Select storage backend for archival data: chroma
? Select chroma backend: persistent
? Select storage backend for recall data: sqlite
📖 Saving config to /Users/loaner/.memgpt/config

% memgpt run
Loading config from /Users/loaner/.memgpt/config

? Would you like to select an existing agent? No

🧬 Creating new agent...
->  🤖 Using persona profile: 'sam_pov'
->  🧑 Using human profile: 'basic'
🎉 Created new agent 'LoyalPumpkin' (id=ab53ba74-6d44-49c4-8d17-4779a2b14f81)

Hit enter to begin (will request first MemGPT message)

💭 I wonder what Chad needs help with. I'm curious to see what kind of conversations we'll have.
🤖 Hello Chad. I'm Sam. How can I help you today?
> Enter your message: my name is brad, not chad
💭 Oops, my mistake. I'll remember that from now on.
⚡🧠 [function] updating memory with core_memory_replace
         First name: Chad
        → First name: Brad
> Enter your message: is anyone there?
💭 I should be more careful with names in the future.
🤖 Yes, I'm here. How can I help you today, Brad?

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Please describe the purpose of this pull request.

Adds support for the Cohere API. Notes:

• Because the Cohere /chat API only supports back-and-forth text messages between CHATBOT and USER, we use the SYSTEM role to interleave function calls and function responses into the conversation history
• The preamble param is used in the payload instead of a SYSTEM role message as chat_history[0]

[sarahwooders]

lgtm!