add API to get swap info

[zoedberg]

Add a new API to get info about a single swap via its hash.

• endpoint: /getswap
• method: POST (needed to pass the parameter in a request body)
• request body: GetSwapRequest { payment_hash: String } (the hash of the swap to get the info for)
• response body: GetSwapResponse { swap: Swap } (Swap structure already exists)
• documentation: add to README.md and openapi.yaml
• hint: check the implementation of /listswaps and /getpayment

[Arshia-r-m]

Hi, I'm interested into resolving this issue

[zoedberg]

Closing this since done in #55