Add ability to buy reservations for instant outs from server

[sputn1ck]

This pull request introduces several new features and improvements related to reservation management and state handling. The most significant changes include adding a new reservation command, updating the reservation state machine to support client-initiated reservations, and enhancing error handling and timeout configurations.

New Features:

• Added a new reservation command to create reservations with specified values and expiry times in cmd/loop/reservations.go. [1] [2] [3]
• Introduced client-initiated reservation handling in instantout/reservation/actions.go and instantout/reservation/fsm.go. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

Enhancements:

• Added a default RPC timeout configuration in cmd/loop/main.go.
• Updated error handling and added timeout configurations for prepayment in instantout/reservation/actions.go. [1] [2] [3]

Testing and Mocking:

• Updated tests and mock implementations to support new reservation commands and client-initiated reservations in instantout/reservation/actions_test.go. [1] [2] [3]

Code Refactoring:

• Refactored the reservation manager to handle asynchronous events and maintain current blockchain height in instantout/reservation/manager.go. [1] [2] [3] [4]

These changes improve the flexibility and robustness of the reservation system, allowing for better error handling and support for both server-initiated and client-initiated reservations.

[lightninglabs-deploy]

@bhandras: review reminder
@starius: review reminder
@hieblmi: review reminder