Add GHA to invoke RPC integration tests on commits to master

[Shaptic]

Description

As part of https://github.com/stellar/pod-fsr/issues/2, we want Stellar Core to run RPC integration tests to catch failures prior to downstream integration.

Checklist

• Reviewed the contributing document
• Rebased on top of master (no merge commits)

Not Applicable:

• Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• Compiles
• Ran all tests
• If change impacts performance, include supporting evidence per the performance document

[Copilot]

Pull request overview

This PR adds a new GitHub Actions workflow (.github/workflows/rpc-integration.yml) that triggers RPC integration tests on every push to the master branch. It does so by invoking a reusable workflow from stellar/stellar-rpc and passing the current commit SHA as the core_git_ref input. This supports the goal of catching downstream integration failures early, before they propagate to RPC.

Changes:

• Adds a new GHA workflow file that calls the RPC integration test suite on every push to master
• Passes github.sha as core_git_ref so that the RPC tests build against the exact Stellar Core commit being merged