feat: add support for LUD-19 (#53)

https://github.com/lnurl/luds/blob/luds/19.md

Author: dni

README.md

@@ -35,7 +35,7 @@ Check out the LUDS repository: [luds](https://github.com/lnurl/luds/)
 - [x] LUD-16
 - [ ] LUD-17
 - [ ] LUD-18
-- [ ] LUD-19
+- [x] LUD-19
 - [x] LUD-20
 - [ ] LUD-21
 

lnurl/models.py

@@ -159,6 +159,8 @@ class LnurlWithdrawResponse(LnurlResponseModel):
     min_withdrawable: MilliSatoshi = Field(..., alias="minWithdrawable", gt=0)
     max_withdrawable: MilliSatoshi = Field(..., alias="maxWithdrawable", gt=0)
     default_description: str = Field("", alias="defaultDescription")
+    # LUD-19: Pay link discoverable from withdraw link.
+    pay_link: Optional[str] = Field(None, alias="payLink")
 
     @validator("max_withdrawable")
     def max_less_than_min(cls, value, values):