Add generated xdr-json schema files

[leighmcculloch]

What

Add generated xdr-json schema files.

Why

So that XDR-JSON schema files can be referenced via a URL for any commit / build / release of the stellar-xdr crate.

So that tooling that embeds the stellar-xdr crate (CLI, Lab, RPC), and supports parsing XDR-JSON and converting from XDR to and from JSON, can also reference a schema URL for IDEs and editors like VS Code that can use JSON schema to auto-complete and provide hints about valid and invalid JSON.

The generated JSON files are checked by CI via the existing generate job, that doesn't need updating to do the checks because it broadly checks for any diff after regeneration.

As a bonus, after this change is merged, any future changes to the XDR will be more clearly evident as to how they impact the XDR-JSON schema.

Known limitations

This change doesn't yet embed a $schema key into JSON generated or parsed yet. It is unclear yet if we will add that here, or not, because not all uses of the XDR-JSON would benefit from it. So applications, like the CLI, that will use it, may do so as needed separately.

This change doesn't result in the JSON being served on a particularly friendly URL, but we will be able to use the stellar.org nginx's ability to redirect paths to redirect a nicer stellar.org URL to the files hosted here.

There's an argument for hosting these files outside this repo, however hosting them here is a trade-off towards simplifying the moving parts that support this.

cc @stellar/devx @fnando

[fnando]

nice!

ps: it's funny how this pr made github unusable. 🫠

[leighmcculloch]

@fnando I was debating limiting the files written to the TransactionEnvelope, at least for the moment, so as not to significantly burden the file system with the +457,000 lines. I'm hoping the GH slowness is just because of the size of the PR, and will not continue once merged.