Change enum serialization to lowercase

[randomlogin]

Changed the serialization macros to lowercase enum names.

Strings like "COOPERATIVE_CLOSURE", "SUCCEEDED", or "SIGNET" look unnatural in CLI output and human-facing JSON, even though SCREAMING_SNAKE_CASE is the proto3 default.

This relies on enum variants following the proto style guide (UPPER_SNAKE_CASE); a future variant using acronyms or mixed case (e.g. HTTPSEndpoint) would lowercase awkwardly. All current variants are clean.

Note: this is a breaking change for any existing JSON consumer of PaymentDirection, PaymentStatus, or BalanceSource.

[ldk-reviews-bot]

👋 Thanks for assigning @benthecarman as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[benthecarman]

tbh I don't really love this, seeing status: SUCCEEDED feels more like an enum whereas status: succeeded feels like someone hard coded a string to me

[randomlogin]

tbh I don't really love this, seeing status: SUCCEEDED feels more like an enum whereas status: succeeded feels like someone hard coded a string to me

Can it be so because you know the internals and the fact that there is an enum?
Funny enough, it makes me feel the contrary: in "naturally occurring" JSONs an uppercase field gives a suspicion that something went wrong in a piece of software and I received a response encoded for a machine and maybe I need to recheck if there was no error.