Make `generate_session_key()` public

[edgerunnergit]

PR Type

Refactor

PR Checklist

• [x] Tests for the changes have been added / updated.
• [x] Documentation comments have been added / updated.
• [x] A changelog entry has been made for the appropriate packages.
• [x] Format code with the nightly rustfmt (cargo +nightly fmt).

Overview

The PR makes the function generate_session_key() public. And changes it's implementation to use DistString::sample_string to get one less unwrap().

Closes https://github.com/actix/actix-extras/issues/434