Webhook Support

[nullstyle]

For personally run instances of horizon, we should provide some sort of ability for the server owner to configure webhook callbacks for the various async events that occur on the stellar network. Some possible notifications:

• Ledger closes
• Closed transactions in which a specified account has been a participant
• Results of a specific transaction submission
• Trades occurring against a specified order boook
• Some third party establishing a trustline to a specified account

Open questions:

• Do these webhooks get configured at runtime? Do they get registered in a config file or environment variable?