Make CommitmentLevel constructors const

[fhartwig]

Problem

The CommitmentConfig constructors cannot be used in const contexts. This isn't a big problem because CommitmentConfigs's only field is public, so sdk users can just use e.g. CommitmentConfig{commitment: CommitmentLevel::Finalized} instead of CommitmentConfig::finalized(), but using the latter would be a bit more convenient and there aren't any obvious-to-me reasons not to make the functions const.

Summary of Changes

Make the constructors const.

Fixes #

[CriesofCarrots]

Sure, I don't see any reason not to do this either. That said, a use-case would have been nice; "no reasons not to" is not the same as a reason to do something :)