Allow customers to add their own custom unit conversion formulas

[drewmiranda-gl]

What?

Graylog 6.1 introduces a great framework that provides tooling for unit conversions in aggregation widgets. It would be useful to allow users to also specify their own custom formulas.

Why?

Graylog will have difficulty meeting the needs of all the possible scenarios for unit conversions, but allowing Graylog admins the ability to configure the formulas that are most important to them, it can work around whatever conversions we may be missing.

This is for a Solutions Consultant customer, please let me know if there are any quesitons