## Title
Optimise `update_validators` by decrypting key cache only when necessary
## Issue Addressed
Resolves [#3968: Slow performance of validator client PATCH API with hundreds of keys](https://github.com/sigp/lighthouse/issues/3968)
## Proposed Changes
1. Add a check to determine if there is at least one local definition before decrypting the key cache.
2. Assign an empty `KeyCache` when all definitions are of the `Web3Signer` type.
3. Perform cache-related operations (e.g., saving the modified key cache) only if there are local definitions.
## Additional Info
This PR addresses the excessive CPU usage and slow performance experienced when using the `PATCH lighthouse/validators/{pubkey}` request with a large number of keys. The issue was caused by the key cache using cryptography to decipher and cipher the cache entities every time the request was made. This operation called `scrypt`, which was very slow and required a lot of memory when there were many concurrent requests.
These changes have no impact on the overall functionality but can lead to significant performance improvements when working with remote signers. Importantly, the key cache is never used when there are only `Web3Signer` definitions, avoiding the expensive operation of decrypting the key cache in such cases.
Co-authored-by: Maksim Shcherbo <max.shcherbo@consensys.net>
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