Add framework for converting XDR structures to JSON

[Shaptic]

What

This PR adds support for bsae64-encoded XDR fields to instead be returned as unpacked JSON objects.

New Request Parameter

We introduce a new, optional parameter to all endpoints with XDR outputs:

xdrFormat?: "base64" | "json"

When omitted, the behavior does not change and we encode fields as base64.

New Response Fields

There are new field names for the JSONified versions of XDR structures. Any field with an Xdr suffix (e.g., resultXdr in getTransaction()) will be replaced with one that has a Json suffix (e.g., resultJson) that is a JSON object verbosely and completely describing the XDR structure.

Certain XDR-encoded fields do not have an Xdr suffix, but those also have a *Json equivalent and are listed below:

• getEvents: topic -> topicJson, value -> valueJson
• getLedgerEntries: key -> keyJson, xdr -> dataJson
• getLedgerEntry: xdr -> entryJson
• simulateTransaction: transactionData, events, results.auth, restorePreamble.transactionData, stateChanges.key|before|after all have a Json suffix, and results.xdr is now results.returnValueJson

Notes for reviewers

• It is much easier to see the changes when you turn whitespace off.
• Start with the bindings layer:
  • lib.rs has the Rust code to perform the XDR -> JSON conversion via stellar_xdr.
  • xdr_json.h just has the header definitions to define what is accessible from Go -> Rust.
  • conversion.go has the actual binding invocation:
    • convertAnyBytes is the one that does the low-level Go -> C/FFI -> Rust work
    • the exported members let other packages use it and one is for validation
    • TransactionToJSON is an isolated way to see how the conversion is done for a transaction structure
    • the others are just helper abstractions
• Then, review one of the endpoints, e.g., get_transaction.go. Each one of these has three sets of changes:
  • Validation for the new request parameter
  • New fields with a Json (serialized) and JSON (Golang) suffix with a 1:1 mapping to their corresponding base64-encoded fields that should always be a json.RawMessage.
  • A switch statement in the handler that uses the new xdr2json package (i.e. conversion.go) to convert XDR structures to JSON.
• The same changes should be in every endpoint.
• Most test changes have a lot of variable renames (not serialized fields), so feel free to ignore those entirely.
• There are also new tests for the JSON behavior.

Why

Closes #124.

Known Limitations

Remaining Items

• Complete the initial implementation
• Add support for all endpoints:
  • getTransaction
  • getTransactions
  • getLedgerEntry
  • getLedgerEntries
  • getEvents
  • sendTransaction
  • simulateTransaction
• Testing for all of the above
• Polish

Alternative Implementations

Would it be possible to extend xdrgen to actually have .JSON()-like methods attached? This would be a significant lift there, since it'd need to do the Go <-> Rust FFI, but the calling code would be far cleaner and we'd get JSON output "for free" across any Golang project that used stellar/go/xdr...

[Shaptic]

@2opremio sure, that's fine with me! In that case, do you mind approving this to merge into a feature branch (if there are no other critiques) so that we can still isolate the preflight <-> xdr2json separation changes?

Actually, it ended up being a lot easier than I expected. Done in bf70f6f!

[tamirms]

LGTM!