Add pallet_external_validators, remove ValidatorManager

[tmpolaczyk]

Add pallet_external_validators, remove pallet ValidatorManager.

Contains a migration to populate initial validators and remove all storage from ValidatorManager pallet.

Main storage items:

• WhitelistedValidators: Fixed validators set by root/governance. Have priority over the external validators.
• ExternalValidators: Validators set using storage proofs from another blockchain. Can be disabled by setting
  SkipExternalValidators to true.

Validators only change once per era. By default the era changes after a fixed number of sessions (currently 1 era = 6 sessions), but new eras can be forced or disabled using a root extrinsic.

[github-actions]

WASM runtime size check:

Compared to target branch

dancebox runtime: 1412 KB (no changes) ✅

flashbox runtime: 832 KB (no changes) ✅

dancelight runtime: 1988 KB (+1988 KB) ⚠️

container chain template simple runtime: 1096 KB (-4212 KB) ✅

container chain template frontier runtime: 1388 KB (-5136 KB) ✅

[github-actions]

Coverage Report

(master)

@@                      Coverage Diff                       @@
##           master   tomasz-external-validators      +/-   ##
==============================================================
+ Coverage   65.25%                       65.31%   +0.06%     
+ Files         302                          304       +2     
+ Lines       52728                        53181     +453     
==============================================================
+ Hits        34403                        34733     +330     
+ Misses      18325                        18448     +123     

Files Changed	Coverage
/pallets/invulnerables/src/lib.rs	87.74% (+1.25%)	✅
/primitives/traits/src/lib.rs	65.57% (+0.08%)	✅
/runtime/common/src/migrations.rs	90.32% (+0.57%)	✅
/runtime/dancebox/src/weights/pallet_treasury.rs	60.29% (+39.70%)	✅
/solo-chains/runtime/dancelight/src/genesis_config_presets.rs	58.95% (+0.59%)	✅
/solo-chains/runtime/dancelight/src/tests/common/mod.rs	96.95% (+0.04%)	✅
/solo-chains/runtime/dancelight/src/tests/migrations_test.rs	97.47% (-2.53%)	❗

Coverage generated Thu Oct 31 17:27:48 UTC 2024

[girazoki]

A couple of things we need here:

• we need to add a certain eraLength concept and increment the era when N number of sessions have elapsed (similar to how polkadot does it in pallet-staking)
• we need a hook OnEraStart that we can implement other pallets to perform when an era starts
• similarly, we need a hook called OnEraEnd

[girazoki]

Overall the design of the pallet looks good to me, I just have some comments related to things I would change in the pallet plus testing, but overall looks good

[girazoki]

LGTM