feat: add configurable rate limits with functional options API

[terwey]

Summary

Adds configurable rate limiting based on 3Commas subscription tiers and refactors client initialization to use functional options pattern instead of config structs.

Changes

Rate Limiting by Plan Tier

• Add PlanTier enum with three tiers matching 3Commas API limits:
  • Starter: 5 requests/minute (read-only)
  • Pro: 50 requests/minute (read-only)
  • Expert: 120 requests/minute (read/write, default)
• Modify rate limiter engine to use tier-based global limits
• Update WithThreeCommasRateLimits() to accept optional tier parameter

Functional Options Pattern

• Replace ClientConfig struct with functional options for cleaner API
• Add new option functions:
  • WithAPIKey(string)
  • WithPrivatePEM([]byte)
  • WithThreeCommasBaseURL(string)
  • WithPlanTier(PlanTier)
• Validate required fields (API key, private key) in constructor

Testing & Documentation

• Update all existing tests to use new option pattern
• Add comprehensive rate limiter tests for each tier
• Update README with:
  • Functional options usage examples
  • Rate limiting configuration guide
  • Tier comparison table
  • Updated features list

Migration Guide

Before:

client, err := threecommas.New3CommasClient(threecommas.ClientConfig{
    APIKey:     "key",
    PrivatePEM: pemBytes,
})

After:

client, err := threecommas.New3CommasClient(
    threecommas.WithAPIKey("key"),
    threecommas.WithPrivatePEM(pemBytes),
    threecommas.WithPlanTier(threecommas.PlanPro), // optional
)

Rationale

3Commas introduced tiered rate limits based on subscription plans. This change allows users to configure their tier to avoid unnecessary rate limiting while maintaining backward compatibility (defaults to Expert tier).