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.

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

crates/assistant_settings/Cargo.toml

@@ -21,6 +21,7 @@ lmstudio = { workspace = true, features = ["schemars"] }
 log.workspace = true
 ollama = { workspace = true, features = ["schemars"] }
 open_ai = { workspace = true, features = ["schemars"] }
+deepseek = { workspace = true, features = ["schemars"] }
 schemars.workspace = true
 serde.workspace = true
 settings.workspace = true