Adds BlockNumberProvider in multisig, proxy and nft pallets

[gupnik]

Step in #3268

This PR adds the ability for these pallets to specify their source of the block number. This is useful when these pallets are migrated from the relay chain to a parachain and vice versa.

This change is backwards compatible:

1. If the BlockNumberProvider continues to use the system pallet's block number
2. When a pallet deployed on the relay chain is moved to a parachain, but still uses the relay chain's block number

However, we would need migrations if the deployed pallets are upgraded on an existing parachain, and the BlockNumberProvider uses the relay chain block number.

[kianenigma]

LGTM as abstractions to add, excluding the possible need for a migration.

[kianenigma]

The metric through which you have chosen to migrate Nft pallet seems wrong btw -- this pallet is not priority as it is not on the relay chain.