Prompt caching for Claude

[tpaulshippy]

What this does

Support prompt caching in both Anthropic and Bedrock providers for Claude models that support it.

Caching system prompts:

chat = RubyLLM.chat
chat.with_instructions("You are a helpful assistant.")
chat.cache_prompts(system: true)

Caching user prompts:

chat = RubyLLM.chat
chat.with_instructions("You are a helpful assistant.")
chat.cache_prompts(system: true, user: true)
chat.ask("What is the capital of France?")

Caching tool definitions:

chat = RubyLLM.chat
chat.with_instructions("You are a helpful assistant.")
chat.with_tool(MyTool)
chat.cache_prompts(tools: true)
chat.ask("What is the capital of France?")

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation
• Performance improvement

Scope check

• I read the Contributing Guide
• This aligns with RubyLLM's focus on LLM communication
• This isn't application-specific logic that belongs in user code
• This benefits most users, not just my specific use case

Quality check

• I ran overcommit --install and all hooks pass
• I tested my changes thoroughly
• I updated documentation if needed
• I didn't modify auto-generated files manually (models.json, aliases.json)

API changes

• Breaking change
• New public methods/classes
• Changed method signatures
• No API changes

Related issues

Resolves #13

[tpaulshippy]

@crmne As I don't have an Anthropic key, I'll need you to generate the VCR cartridges for that provider. Hoping everything just works, but let me know if not.