fix: allow unbonded validator to submit heartbeats

[milapsheth]

Description

Due to a change in #1198 for missed blocks, we started returning an error for any validator whose signing info wasn't found. This includes any validator who's newly created but hasn't joined the active set yet. Also, any validator who gets unbonded will have missed blocks. Since we want to allow validators who are unbonding/unbonded to still sign for existing key IDs, we shouldn't exclude them from based on this illegibility.

This PR changes the missed blocks check to only be applicable for bonded validators.

Alternative approaches:

• Filter sign illegibility for missed blocks, like we do for proxy deregistered. But this filter will also apply to bonded validators who are missing blocks, so it would require some refactoring.
• Include all validators from the snapshot in sign, due to robustness. This will be an issue for non robust threshold schemes later. And if they don't respond, we might suspend validators unnecessarily.

Todos

• Unit tests
• Manual tests
• Documentation
• Connect epics/issues
• Tag type of change

Steps to Test

Expected Behaviour

Other Notes

[jcs47]

LGTM