feat: deprecate `initial` in favor of `multiplier` in `wait_exponential_jitter` (#629)

Every other exponential wait class uses `multiplier`. Having `initial`
only on `wait_exponential_jitter` is confusing. Add `multiplier` param
and deprecate `initial` as its alias.

- Add `multiplier` parameter (default 1)
- Emit `DeprecationWarning` when `initial` is used
- Raise `ValueError` if both `initial` and `multiplier` are specified


Change-Id: I63d23f41e58627085118b633a009c86ff2807f79

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>