fix(json schema): manually implement generic types

[willemneal]

This creates a JsonSchema impl for VecM, BytesM, Frame, and StringM and ensures that they are in-lined.

[willemneal]

@leighmcculloch I updated to use base64 since there doesn't seem to be hex support, but base64 is supported.

[leighmcculloch]

Do you mean that json schemas in general don't support hex, or the lib we're using doesn't?

The JSON rendered from the xdr lib is already outputting hex today, not base64.

[willemneal]

JSON schema the spec provides a "format" property for strings. Is hex for historical reasons? We can also allow for both.

[leighmcculloch]

I think contentEncoding would be more appropriate than format, but yeah neither support hex in the specification.

[leighmcculloch]

A similar issue is that strings generated are printable-ASCII escaped strings, not UTF-8 encoded. For example, the following is valid XDR for an ScVal::String:

AAAADgAAAARow2V5

It contains a four character string where the hex values are for those four characters:

$ echo -n 'AAAADgAAAARow2V5' | base64 -d | hexdump -C
00000000  00 00 00 0e 00 00 00 04  68 c3 65 79              |........h.ey|
0000000c

The second character is 0xc3 which is not valid UTF-8.

It decodes to the following JSON:

$ echo -n 'AAAADgAAAARow2V5' | soroban lab xdr decode --type ScVal
{"string":"h\\xc3ey"}

[leighmcculloch]

I think the above means the JSON generated can't be completely described by a JSON schema, but maybe that's okay. It can still be described sufficiently for a lot of uses? Applications will just need to figure out somehow that certain fields result in certain types of data?

Can we use a non-standard contentEncoding or format?

[leighmcculloch]

Is hex for historical reasons?

Hex is because most binary values in the XDR are more commonly displayed as hex in UIs, things like hashes, or 32-byte IDs.

But a reason not to change it is this JSON is in use, so changing it would be breaking and potentially disruptive. It'd be best not to change the output unless there's a big win involved.

[leighmcculloch]

Closing as this has been merged into stellar/xdrgen#193.