Make `IdentityInfo` generic in `pallet-identity` (#1661)

Fixes #179 

# Description

This PR makes the structure containing identity information used in
`pallet-identity` generic through the pallet `Config`. Additionally, the
old structure is now available in a separate module called `simple`
(pending rename) and is compatible with the new interface.

Another change in this PR is that while the `additional` field in
`IdentityInfo` stays for backwards compatibility reasons, the associated
costs are stil present in the pallet through the `additional` function
in the `IdentityInformationProvider` interface. This function is marked
as deprecated as it is only a temporary solution to the backwards
compatibility problem we had. In short, we could have removed the
additional fields in the struct and done a migration, but we chose to
wait and do it off-chain through the genesis of the system parachain.
After we move the identity pallet to the parachain, additional fields
will be migrated into the existing fields and the `additional` key-value
store will be removed. Until that happens, this interface will provide
the necessary information to properly account for the associated costs.

Additionally, this PR fixes an unrelated issue; the `IdentityField` enum
used to represent the fields as bitflags couldn't store more than 8
fields, even though it was marked as `#[repr(u64)]`. This was because of
the `derive` implementation of `TypeInfo`, which assumed `u8` semantics.
The custom implementation of this trait in
https://github.com/paritytech/polkadot-sdk/commit/0105cc03


fixes the issue.

---------

Signed-off-by: georgepisaltu <george.pisaltu@parity.io>
Co-authored-by: Sam Johnson <sam@durosoft.com>
Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com>