Consider making BLAS thread setting optional, RhpcBLASctl Suggests

[MichaelChirico]

Follow-up to 959c9d6.

{RhpcBLASctl} is AGPL-licensed -- that makes it unattractive for many reasons, e.g.

https://opencoreventures.com/blog/2023-10-agpl-license-is-a-non-starter-for-most-companies/
https://deavid.wordpress.com/2020/08/02/affero-gpl-is-toxic-avoid-it-like-the-plague/
https://opensource.google/documentation/reference/using/agpl-policy

By having {RhpcBLASctl} in Imports, that means {mlr3} also can't be used without patching to avoid the AGPL dependency.

Glancing at the codebase, I think we can reasonably move that code to set BLAS threads as optional, and thereby relegate {RhpcBLASctl} to a Suggests dependency. WDYT?

Happy to file the relevant PR.