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.lock

@@ -560,6 +560,7 @@ version = "0.1.0"
 dependencies = [
  "anthropic",
  "anyhow",
+ "deepseek",
  "feature_flags",
  "fs",
  "gpui",
@@ -3685,6 +3686,18 @@ dependencies = [
  "winapi",
 ]
 
+[[package]]
+name = "deepseek"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+ "futures 0.3.31",
+ "http_client",
+ "schemars",
+ "serde",
+ "serde_json",
+]
+
 [[package]]
 name = "deflate64"
 version = "0.1.9"
@@ -6808,6 +6821,7 @@ dependencies = [
  "anyhow",
  "base64 0.22.1",
  "collections",
+ "deepseek",
  "futures 0.3.31",
  "google_ai",
  "gpui",
@@ -6851,6 +6865,7 @@ dependencies = [
  "client",
  "collections",
  "copilot",
+ "deepseek",
  "editor",
  "feature_flags",
  "fs",