add API to decode a swapstring

[zoedberg]

Add a new API to decode a swapstring.

• endpoint: /decodeswapstring
• method: POST (needed to pass the parameter in a request body)
• request body: DecodeSwapstringRequest { swapstring: String } (the swapstring to decode)
• response body:

pub(crate) struct DecodeSwapstringResponse {
    pub(crate) qty_from: u64,
    pub(crate) qty_to: u64,
    pub(crate) from_asset: Option<String>,
    pub(crate) to_asset: Option<String>,
    pub(crate) expiry: u64,
    pub(crate) payment_hash: String,
}

• documentation: add to README.md and openapi.yaml
• hint: check SwapString::from_str

[moakilodash]

i will resolve this issue.