The fields were missing because they weren't annotated with a type and
a description. Adding those fixes them.
Changelog-Fixed: msggen: `listpays` now includes the missing `amount_msat` and `amount_sent_msat` fields
No-schema-diff-check: fields were always there, just undocumented!
This adds our first (basic) schema, and sews support into pyln-testing
so it will load schemas for any method for doc/schemas/{method}.schema.json.
All JSON responses in a test run are checked against the schema (if any).
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
