Add DeepSeek support (#23551)

- Added support for DeepSeek as a new language model provider in Zed
Assistant
- Implemented streaming API support for real-time responses from
DeepSeek models.
- Added a configuration UI for DeepSeek API key management and settings.
- Updated documentation with detailed setup instructions for DeepSeek
integration.
- Added DeepSeek-specific icons and model definitions for seamless
integration into the Zed UI.
- Integrated DeepSeek into the language model registry, making it
available alongside other providers like OpenAI and Anthropic.

Release Notes:

- Added support for DeepSeek to the Assistant.

---------

Co-authored-by: Marshall Bowers <git@maxdeviant.com>

Cargo.toml

@@ -32,6 +32,7 @@ members = [
     "crates/copilot",
     "crates/db",
     "crates/diagnostics",
+    "crates/deepseek",
     "crates/docs_preprocessor",
     "crates/editor",
     "crates/evals",
@@ -229,6 +230,7 @@ context_server = { path = "crates/context_server" }
 context_server_settings = { path = "crates/context_server_settings" }
 copilot = { path = "crates/copilot" }
 db = { path = "crates/db" }
+deepseek = { path = "crates/deepseek" }
 diagnostics = { path = "crates/diagnostics" }
 editor = { path = "crates/editor" }
 extension = { path = "crates/extension" }