Collecting validator verification key in Substrate - generic way

[yahortsaryk]

Description

This PR removes the excessive set_current_validator extrinsic and applies the generic approach for collecting the verification key from the keystore as it is done in the substrate source code

Types of Changes

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• Hotfix
• Release
• Fix or Feature

Fix or Feature

Types of Changes

• Tech Debt (Code improvements)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Dependency upgrade (A change in substrate or any 3rd party crate version)

Migrations and Hooks

• This change requires a runtime migration.
• Modifies on_initialize
• Modifies on_finalize

Checklist for Fix or Feature

• Change has been tested locally.
• Change adds / updates tests if applicable.
• Changelog doc updated.
• spec_version has been incremented.
• network-relayer's events have been updated according to the blockchain events if applicable.
• All CI checks have been passed successfully

Checklist for Hotfix

• Change has been deployed to Testnet.
• Change has been tested in Testnet.
• Changelog has been updated.
• Crate version has been updated.
• spec_version has been incremented.
• Transaction version has been updated if required.
• Pull Request to dev has been created.
• Pull Request to staging has been created.
• network-relayer's events have been updated according to the blockchain events if applicable.
• All CI checks have been passed successfully

Checklist for Release

• Change has been deployed to Devnet.
• Change has been tested in Devnet.
• Change has been deployed to Qanet.
• Change has been tested in Qanet.
• Change has been deployed to Testnet.
• Change has been tested in Testnet.
• Changelog has been updated.
• Crate version has been updated.
• Spec version has been updated.
• Transaction version has been updated if required.
• All CI checks have been passed successfully

[yahortsaryk]

This upgrade should be merged and deployed to DEVNET after the #457 is deployed. That's why I'm removing the related migrations here.

[khssnv]

There is a similar pattern in authority-discovery, babe, and such. But they define AuthorityId as an alias for the Public made by the macros (https://github.com/paritytech/polkadot-sdk/blob/c8d2251cafadc108ba2f1f8a3208dc547ff38901/substrate/primitives/authority-discovery/src/lib.rs#L35). Should we do the same for validators accounts or the AccountId suits for us?