feat: migrate backend from rust to go

This commit replaces the Axum/Rust backend with a Gin/Go implementation. The original Rust code has been archived in the 'rust' branch.

internal/providers/provider.go

@@ -0,0 +1,13 @@
+package providers
+
+import (
+	"context"
+
+	"llm-proxy/internal/models"
+)
+
+type Provider interface {
+	Name() string
+	ChatCompletion(ctx context.Context, req *models.UnifiedRequest) (*models.ChatCompletionResponse, error)
+	ChatCompletionStream(ctx context.Context, req *models.UnifiedRequest) (<-chan *models.ChatCompletionStreamResponse, error)
+}