[Identity] Decouple usernames from identities (#5554)

This PR refactors `pallet-identity` to decouple usernames from
identities.

Main changes in this PR:
- Separate usernames from identities in storage, allowing for correct
deposit accounting
- Introduce the option for username authorities to put up a deposit to
issue a username
- Allow authorities to remove usernames by declaring the intent to do
so, then removing the username after the grace period expires
- Refactor the authority storage to be keyed by suffix rather than owner
account.
- Introduce the concept of a system provider for a username, different
from a governance allocation, allowing for usernames set by the system
and not a specific authority
- Implement multi-block migration to enable all of the changes described
above

---------

Signed-off-by: georgepisaltu <[email protected]>
Co-authored-by: Ankan <[email protected]>