Add multi-platform XDP CI builds (Alveo, Edge, VE2)

[snigdha-gupta]

Summary

Extends the XDP GitHub Actions workflow to build XRT with the PR’s XDP overlay on Alveo, Edge, and VE2 when a pull request targets master.

• Alveo: native build on GitHub-hosted ubuntu-latest; validates libxrt_core and libxdp_core artifacts
• Edge: Versal aarch RPM build on self-hosted runner
• VE2: Telluride/Yocto RPM build on self-hosted runner with a persistent workspace cache (ci/ve2/ensure_yocto_cache.sh, 7-day TTL)
• Label-based builds: optional selective runs via build-xrt-alveo, build-xrt-edge, build-xrt-ve2, or build-xrt-client; default (no labels) runs Alveo + Edge + VE2

Each build job uploads artifacts and writes a step summary.

Infrastructure

• VE2 Yocto cache path is configured on the self-hosted runner (VE2_YOCTO_* env vars on the VE2 job)
• Client build remains a placeholder; runs only when build-xrt-client is applied

Out of scope

Board testing is not included in this PR.