feat: LLM SSE MCP support

[zees-dev]

Description

This pull request includes several changes to the blockless-sdk project, focusing on the addition of a new example, updates to the SupportedModels enum, and enhancements to the LlmOptions struct.
Below is a summary of the most important changes:

New Example Addition:

• Added a new example llm-mcp demonstrating the use of the Blockless SDK to interact with LLM models and use MCP for tool integration via SSE endpoints (README.md, examples/llm-mcp.rs). [1] [2]

Enhancements to SupportedModels Enum:

• Added a Custom variant to the SupportedModels enum to support custom models (src/llm.rs). [1] [2] [3]

Enhancements to LlmOptions Struct:

• Modified the LlmOptions struct to include tools_sse_urls and updated methods to handle optional fields (src/llm.rs). [1] [2]
• Updated the set_options method calls in examples/llm.rs to use the new with_system_message method (examples/llm.rs). [1] [2]

Version Update:

• Updated the package version from 0.1.8 to 0.1.9 in Cargo.toml to reflect the new changes (Cargo.toml).