All delegations rpc

[aramikm]

Goal

The goal of this PR is to expose an RPC which would return all providers for an msa

Closes #2019

Checklist

• [x] Chain spec updated
• [x] Custom RPC OR Runtime API added/changed? Updated js/api-augment.
• [x] Tests added

Verification

• Calling the RPC returns expected result

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 85.29412% with 5 lines in your changes missing coverage. Please review.

Files	Coverage Δ
pallets/msa/src/lib.rs	88.07% <100.00%> (+0.17%)	:arrow_up:
pallets/msa/src/rpc/src/lib.rs	90.74% <100.00%> (+1.15%)	:arrow_up:
common/primitives/src/msa.rs	83.96% <37.50%> (-3.80%)	:arrow_down: