Slack/Telegram notification support

[BaumerCrypto]

Add support for Slack and Telegram notifications alongside Discord.

Approach:

• Config file specifies which backends are enabled
• Each backend has its own send function
• Alerts fire to all enabled backends

# ~/.oracle-monitor/config
DISCORD_WEBHOOK="https://..."
SLACK_WEBHOOK="https://..."
TELEGRAM_BOT_TOKEN="..."
TELEGRAM_CHAT_ID="..."

Lets oracle operators use whatever notification platform they prefer.

Version: v2.x