Reorganise relayer config and add retry logic

[salman01zp]

Summary of changes

• added lc-relay-config crate (configuration required by lc-relayer)
• added lc-relay-context crate (structure containing config and shutdown signals)
• Graceful shutdown of lc relayer
• Restart lc-relayer on if substrate connection drops

Reference issue to close (if applicable)

• Closes [TASK] Implement retry logic with backoff for async calls #67
• Closes [TASK] Refactor code to handle substrate node connection drop #71
• Closes [TASK] Create relayer context struct to store relayer config and shutdown signals #77

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Code Checklist

• Tested
• Documented

[drewstone]

LGTM