Proper Priority for Offchain Phragmen Solution

[kianenigma]

Modules that manually validate transaction might want to have a configurable Get<u64> as an indicator of how to set the priority. An example would be staking in which the priority of offchain phragmen solutions is better to be configurable.

To demonstrate the problem, consider this: currently, score[0] is the priority, which makes perfect sense if the pool wants to prioritise between 3 transactions of the same type (submit_election_solution_unsigned). Yet, if there are other insignificant transactions in the pool that happen ho have a less priority, then this becomes problematic and the submission of the solution might get deferred.

[kianenigma]

closed via #5563