Investigate `mapValues` type mapping issue

[nflaig]

Current implementation or rather the typing / generics of mapValues function to seem to be incorrect, requiring to type cast the returned value (although the implementation itself works as expected) https://github.com/ChainSafe/lodestar/blob/c63509800c61eb44e59d41187bb272c0d1e1e462/packages/api/src/utils/client/client.ts#L124

This is the current implementation which needs to be reviewed / reworked

https://github.com/ChainSafe/lodestar/blob/8b6ecc44c1892115d0e7396b2a5f0ae8fc0d9e6f/packages/utils/src/objects.ts#L67-L76

See previous discussion for more context https://github.com/ChainSafe/lodestar/pull/6749#discussion_r1609906528