[DOC] Add Enqueue Outgoing Payment HTTP API Docs.

[DOC] Fix typo.
[TEST] POST /payements/outgoing for  enqueueOutgoingPayment
[FEATURE] Implement controller for enqueue outgoing payment.
[FEATURE] Add validations to api.enqueueOutgoignPayment.
Merge branch 'enqueue-outgoing-payment' of github.com:ripple/gatewayd into enqueue-outgoing-payment