Add GitHub workflow for Valgrind

[thsfs]

This workflow is set to run for every created tag that starts with V. So, it is intended to generate reports for the automatic beta builds and also for release candidates and official releases.

This is set up to run upon core_test and rpc_test on self-hosted runners by default, as these tests are heavy and require higher use of CPU. It is also possible to trigger it to run manually.

The self-hosted label for it is self-hosted-standard that is intended to run on machines that won't be used for benchmarks.

[thsfs]

Will mark this as ready for review when the runner setup is finished.

[dsiganos]

This PR depends on this other PR on another repo: "Add a workflow to automatic respawn the GH self-hosted runners" https://github.com/nanocurrency/infrastructure/pull/102

However, that other PR needs reworking in my opinion. And ideally, this workflow will not depend on a workflow of another repo.