[Backend] Decode Soroban transaction result XDR

[emynogithub]

Summary

• add a reusable decoder for resultMetaXdr, standalone Soroban metadata, and transaction envelopes
• expose decoded contract invocations, events, diagnostics, return values, and resource-fee breakdowns
• document the decoder API and add XDR round-trip/error tests

Validation

• git diff --check
• Rust tests could not be run because this environment does not have cargo installed.

Closes #569

[drips-wave]

@emynogithub Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits