Const fn for GovernorConfigBuilder<PeerIpKeyExtractor>

[TobiasDeBruijn]

This PR adds several const fn's for GovernorConfigBuilder<PeerIpKeyExtractor>, namely:

• const_default
• const_period
• const_per_second
• const_per_millisecond
• const_per_nanosecond
• const_burst_size I was not able to create a const fn for methods, key_extractor and finish. The const fn's were not added in the already existing impl due to trait bounds other than Sized on const fn parameters are unstable. The other three methods do not have a const implementation because involved items (e.g. Vec) requires Drop, Option::unwrap, or uses external functions which are not const.

The implementation of GovernorConfigBuilder<PeerIpKeyExtractor>::default() now calls const_default

Thanks

[TobiasDeBruijn]

Not sure why those other commits were referenced, as those were already implemented in a previous PR :thinking: If wanted I can open a new PR, hopefully without those commits referenced

[AaronErhardt]

Looks good to me. Thanks!