RUST-2369 Add constructor methods for ReadPreference

[OnjoujiToki]

Hi here, playing Mongo for a while so trying to get exposure to driver side's world as well.
Picking some small wins here: RUST-2369 if you don't mind.

This pr adds public constructor methods to ReadPreference so users can build variants without the verbose struct syntax.
Before:

let rp = ReadPreference::PrimaryPreferred { options: None };

After:

let rp = ReadPreference::primary_preferred(None);
let rp = ReadPreference::primary_preferred(options);

thanks for review in advance.
if I've missed any convention here, please let me know and I'll fix it right away.

[abr-egn]

Thank you, this is a nice ergonomic improvement! Can you please rebase to pick up 22ee8fd, the tests won't pass without that.

[OnjoujiToki]

Thanks for your review! @abr-egn
Sorry for the late reply -- wasn't a wfh day for me. Just rebased onto that commit. Out of curiosity, is there a specific reason for rebasing onto that commit rather than the latest merged one? Still getting familiar with the testing setup here, so apologies if this is a naive question!

[abr-egn]

Out of curiosity, is there a specific reason for rebasing onto that commit rather than the latest merged one? Still getting familiar with the testing setup here, so apologies if this is a naive question!

That was the one that fixed the tests is all, rebasing to latest would also have been fine 🙂

[OnjoujiToki]

got it! thanks!