New collective to accept and reject treasury spending proposal

[nanocryk]

What does it do?

Add a new TreasuryCouncilCollective pallet to manage the treasury. It replaces the normal Council for treasury management only. The normal Council is still associated with democracy for exemple.

I added in the tests this new pallet and multiple pallets that were not testecd (for pallet index and prefix). I also encountered compiler ICE while trying to cargo check when some changes were missing. I was able to fix the ICE by bumping the Rust version and find what was missing. Note that after fixing the compilation errors I was able to go back to the previous Rust version and not encounter an ICE, but it should not be a problem to stay with this version and will avoid similar issues in the future.

What important points reviewers should know?

Is there something left for follow-up PRs?

What alternative implementations were considered?

Are there relevant PRs or issues in other repositories (Substrate, Polkadot, Frontier, Cumulus)?

What value does it bring to the blockchain users?