frame-system: Add `last_runtime_upgrade_spec_version` (#2351)

Adds a function for querying the last runtime upgrade spec version. This
can be useful for when writing runtime level migrations to ensure that
they are not executed multiple times. An example would be a session key
migration.

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Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>