contender GitHub Action

**Is your feature request related to a problem? Please describe.**

High-level, I'm thinking for EL nodes' CI, you could just have a one- or two-line addition to your workflow that spams your node with contender and returns relevant metrics in a workflow-compatible format.

**Describe the solution you'd like**

- standalone github action that takes parameters to connect to an EL node (it must be running in the job; running EL nodes via this action is out of scope)
- also take parameters to customize the spam parameters, but set defaults
- allow a file to be passed in that specifies all the parameters used to run the job