Add Blossom package with core functionalities for blob storage and authorization

- Introduced the Blossom package, implementing essential features for handling blob storage, including upload, retrieval, and deletion of blobs.
- Added authorization mechanisms for secure access to blob operations, validating authorization events based on Nostr standards.
- Implemented various HTTP handlers for managing blob interactions, including GET, HEAD, PUT, and DELETE requests.
- Developed utility functions for SHA256 hash calculations, MIME type detection, and range request handling.
- Established a storage layer using Badger database for efficient blob data management and metadata storage.
- Included placeholder implementations for media optimization and payment handling, setting the groundwork for future enhancements.
- Documented the new functionalities and usage patterns in the codebase for better maintainability and understanding.

pkg/blossom/payment.go

@@ -0,0 +1,53 @@
+package blossom
+
+import (
+	"net/http"
+)
+
+// PaymentChecker handles payment requirements (BUD-07)
+type PaymentChecker struct {
+	// Payment configuration would go here
+	// For now, this is a placeholder
+}
+
+// NewPaymentChecker creates a new payment checker
+func NewPaymentChecker() *PaymentChecker {
+	return &PaymentChecker{}
+}
+
+// CheckPaymentRequired checks if payment is required for an endpoint
+// Returns payment method headers if payment is required
+func (pc *PaymentChecker) CheckPaymentRequired(
+	endpoint string,
+) (required bool, paymentHeaders map[string]string) {
+	// Placeholder implementation - always returns false
+	// In a real implementation, this would check:
+	// - Per-endpoint payment requirements
+	// - User payment status
+	// - Blob size/cost thresholds
+	// etc.
+	
+	return false, nil
+}
+
+// ValidatePayment validates a payment proof
+func (pc *PaymentChecker) ValidatePayment(
+	paymentMethod, proof string,
+) (valid bool, err error) {
+	// Placeholder implementation
+	// In a real implementation, this would validate:
+	// - Cashu tokens (NUT-24)
+	// - Lightning payment preimages (BOLT-11)
+	// etc.
+	
+	return true, nil
+}
+
+// SetPaymentRequired sets a 402 Payment Required response with payment headers
+func SetPaymentRequired(w http.ResponseWriter, paymentHeaders map[string]string) {
+	for header, value := range paymentHeaders {
+		w.Header().Set(header, value)
+	}
+	w.WriteHeader(http.StatusPaymentRequired)
+}
+