Add GenesisConfig to identity pallet

[grw-ms]

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Description

This change adds a GenesisConfig to the identity pallet, allowing downstream users to automatically seed Identities via chainspec config.

Checklist

• [x] My PR includes a detailed description as outlined in the "Description" section above
• [x] My PR follows the labeling requirements of this project (at minimum one label for each A, B, C and D required)
• [ ] I have made corresponding changes to the documentation (if applicable)
• [x] I have added tests that prove my fix is effective or that my feature works (if applicable)
• [ ] If this PR alters any external APIs or interfaces used by Polkadot, the corresponding Polkadot PR is ready as well as the corresponding Cumulus PR (optional)

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[paritytech-cicd-pr]

The CI pipeline was cancelled due to failure one of the required jobs. Job name: cargo-check-benches Logs: https://gitlab.parity.io/parity/mirrors/substrate/-/jobs/3406456

[paritytech-cicd-pr]

The CI pipeline was cancelled due to failure one of the required jobs. Job name: cargo-check-each-crate-macos Logs: https://gitlab.parity.io/parity/mirrors/substrate/-/jobs/3406473