feat(heartbeat): add average and stddev functions to heartbeat agg

[ollz272]

This would be useful for us to have for our own usage (We want to use the stats on average uptime/downtime to do some more analysis).

Let me know if anymore is needed.

[CLAassistant]

All committers have signed the CLA.

[syvb]

Please do cargo fmt so that we have consistent formatting - thanks!

[jonatas]

Hi @ollz272, very good, I'm not a Rust dev here, but I'm an avid user of toolkit too 🤗

1. can we add a few more tests just calling the functions and asserting the values?
2. or at least add it on docs/test_caggs that could cover and exemplify.
3. We also have this tests/update/heartbeat.md that could include some tests.

We still don't have the docs/heartbeat_agg which would be the right place to document it. Feel free to introduce the file if you think it makes sense.