Add support for new fields in Soroban RPC's `sendTransaction` endpoint

[Shaptic]

Soroban RPC Changes

With the introduction of stellar/soroban-tools#1152, the Soroban RPC server introduces the following changes:

sendTransaction

The response will include a new, optional field in its response schema:

• diagnosticEventsXdr: string[] - If the transaction status is "ERROR", this field may be present with an array of base64-encoded strings. Each string will decode to a raw DiagnosticEvent XDR struct containing details on why stellar-core rejected the transaction.

SDKs should add support for this field.

References

• The documentation PR: soroban-docs#700.
• An example implementation in JavaScript: stellar-sdk#905.