Allow copying fields when using WrapCore

[adrianlungu]

Is your feature request related to a problem? Please describe.

https://go.dev/play/p/vlsJxJfxhUL

Any fields added via the config, or via With to the logger before calling zap.WrapCore are not seen at all by the observer via the context.

Describe the solution you'd like

After building a logger with fields using .Build(), allow using zap.WrapCore to be able to split, or replace the core of a logger, but keep the fields that were added via the configuration, i.e.

This could be done either via exposing Context on core, or internally by copying the context from one core to the other, although allowing the developer to choose the scenario that fits him best would be better.

Describe alternatives you've considered

None so far that wouldn't involve heavy refactoring.

Is this a breaking change?

It should not be a breaking change.

Additional context
N/A/