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/adaptor.go

@@ -3,13 +3,14 @@ package anthropic
 import (
 	"errors"
 	"fmt"
+	"io"
+	"net/http"
+
 	"github.com/gin-gonic/gin"
 	"github.com/songquanpeng/one-api/relay/channel"
 	"github.com/songquanpeng/one-api/relay/channel/openai"
 	"github.com/songquanpeng/one-api/relay/model"
 	"github.com/songquanpeng/one-api/relay/util"
-	"io"
-	"net/http"
 )
 
 type Adaptor struct {