Move pick_random_relay_excluding to helpers module for consistency

[faern]

All other helper methods for selecting random relays are there. So I thought it would be easier to read if we have them all in one place. Renaming the function to align more consistently with the others at the same time.

I also added tiny amount of docs. And "fixed" a lifetime nitpick.

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This change is 

[faern]

Reviewable status: 0 of 2 files reviewed, all discussions resolved

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mullvad-relay-selector/src/relay_selector/helpers.rs line 35 at r1 (raw file):

pub fn pick_random_relay_excluding<'a>(
    relays: &'a [Relay],
    exclude: &'_ Relay,

Nit, FYI: The relay used for exclusion is not part of the return value. So the exclude reference only has to be valid for the lifetime of this function, not for as long as the return value is alive.