Implement perplexity agent tool

This PR enables AI Agents to use perplexity AI in order to retrieve data
from the web.

tools/perplexity/README.md

@@ -0,0 +1,62 @@
+
+# Perplexity Tool Integration for Agents
+
+Use perplexity in your AI Agent to enrich it with data from the web.
+
+Full code example:
+
+```go
+package main
+
+import (
+	"context"
+	"fmt"
+	"os"
+
+	"github.com/tmc/langchaingo/agents"
+	"github.com/tmc/langchaingo/callbacks"
+	"github.com/tmc/langchaingo/chains"
+	"github.com/tmc/langchaingo/llms/openai"
+	"github.com/tmc/langchaingo/tools"
+	"github.com/tmc/langchaingo/tools/perplexity"
+)
+
+func main() {
+	if err := run(); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+}
+
+func run() error {
+	llm, err := openai.New(
+		openai.WithModel("gpt-4o-mini"),
+		openai.WithCallback(callbacks.LogHandler{}),
+	)
+	if err != nil {
+		return err
+	}
+
+	perpl, err := perplexity.NewPerplexity(perplexity.ModelLlamaSonarSmall)
+	if err != nil {
+		return err
+	}
+
+	agentTools := []tools.Tool{
+		perpl,
+	}
+
+	agent := agents.NewOneShotAgent(llm,
+		agentTools,
+		agents.WithMaxIterations(2),
+	)
+	executor := agents.NewExecutor(agent)
+
+	question := "what's the latest and best LLM on the market at the moment?"
+	answer, err := chains.Run(context.Background(), executor, question)
+
+	fmt.Println(answer)
+
+	return err
+}
+```

tools/perplexity/perplexity.go

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+package perplexity
+
+import (
+	"context"
+	"fmt"
+	"os"
+
+	"github.com/tmc/langchaingo/llms"
+	"github.com/tmc/langchaingo/llms/openai"
+)
+
+type Model string
+
+// Model pricing overview: https://docs.perplexity.ai/guides/pricing
+const (
+	ModelLlamaSonarSmall Model = "llama-3.1-sonar-small-128k-online"
+	ModelLlamaSonarLarge Model = "llama-3.1-sonar-large-128k-online"
+	ModelLlamaSonarHuge  Model = "llama-3.1-sonar-huge-128k-online"
+)
+
+type Perplexity struct {
+	llm *openai.LLM
+}
+
+func NewPerplexity(model Model) (*Perplexity, error) {
+	perplexity := &Perplexity{}
+	var err error
+
+	apiKey := os.Getenv("PERPLEXITY_API_KEY")
+	if apiKey == "" {
+		return nil, fmt.Errorf("PERPLEXITY_API_KEY not set")
+	}
+
+	perplexity.llm, err = openai.New(
+		openai.WithModel(string(model)),
+		openai.WithBaseURL("https://api.perplexity.ai"),
+		openai.WithToken(apiKey),
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	return perplexity, nil
+}
+
+func (p *Perplexity) Name() string {
+	return "PerplexityAI"
+}
+
+func (p *Perplexity) Description() string {
+	return "Perplexity AI has access to a wide range of information, as it functions as an AI-powered search engine that indexes, analyzes, and summarizes content from across the internet."
+}
+
+func (p *Perplexity) Call(ctx context.Context, input string) (string, error) {
+	content := []llms.MessageContent{
+		llms.TextParts(llms.ChatMessageTypeHuman, input),
+	}
+
+	var generatedText string
+	_, err := p.llm.GenerateContent(ctx, content,
+		llms.WithStreamingFunc(func(_ context.Context, chunk []byte) error {
+			generatedText += string(chunk)
+			return nil
+		}))
+	if err != nil {
+		return "", err
+	}
+
+	return generatedText, nil
+}