xdr2json: add the JSON-to-XDR direction for getEvents v2 xdrInputFormat

[urvisavla]

The getEvents v2 request carries xdrInputFormat. The json value selects the JSON encoding for topic values inside filters. We cannot serve it today.

The term index keys each topic term on the canonical XDR byte encoding of the ScVal, so a JSON topic value must convert to those exact bytes. The repository has no converter for that direction. internal/xdr2json is cgo over a Rust staticlib that exposes only xdr_to_json, and the Go SDK's xdr.ScVal has no UnmarshalJSON.

Requirement: a JSON-to-XDR converter for ScVal, so getEvents v2 accepts xdrInputFormat: "json".

Parent issue: #774
Spec: https://github.com/orgs/stellar/discussions/1872

[roebee]

Three questions:

1. Where does the JSON-to-ScVal converter live: a new Go-native package, or an extension of the cgo Rust staticlib in xdr2json?
2. Error contract: which JSON-RPC error code for an invalid JSON topic value, -32602 (invalid params) or -32000 (server error)?
3. Scope: is the converter only for ScVal (topics), or do you need a general JSON-to-XDR path for the v2 API?