refactor: Refactor: Consolidate Anthropic model requests into GeneralOpenAIRequest

- Refactor Anthropic adapter to work with the new Anthropic API and model requests
- Remove the default value for `MaxTokensToSample`
- Set `MaxTokens` to 500 instead of 1000000
- Use `system` messages as the system prompt instead of the first message

relay/channel/anthropic/model.go

@@ -1,19 +1,17 @@
 package anthropic
 
+import (
+	"github.com/songquanpeng/one-api/relay/model"
+)
+
 type Metadata struct {
 	UserId string `json:"user_id"`
 }
 
 type Request struct {
-	Model             string   `json:"model"`
-	Prompt            string   `json:"prompt"`
-	MaxTokensToSample int      `json:"max_tokens_to_sample"`
-	StopSequences     []string `json:"stop_sequences,omitempty"`
-	Temperature       float64  `json:"temperature,omitempty"`
-	TopP              float64  `json:"top_p,omitempty"`
-	TopK              int      `json:"top_k,omitempty"`
-	//Metadata    `json:"metadata,omitempty"`
-	Stream bool `json:"stream,omitempty"`
+	model.GeneralOpenAIRequest
+	// System anthropic messages API use system to represent the system prompt
+	System string `json:"system"`
 }
 
 type Error struct {