Allow `--local-crds` flag to receive multiple values

[doom]

What would you like to be added?

The --local-crds flag currently supports a single value, which restricts loading the CRDs to a single directory.

As a result, invoking kubectl-validate like so will only consider CRDs from path/to/second/dir:

kubectl-validate --local-crds path/to/first/dir --local-crds path/to/second/dir path/to/resources/*.yaml

We could allow this flag to receive multiple values to allow collecting CRDs from multiples directories.

Why is this needed?

The current behavior requires users to have all their CRDs in a single directory, which might require an extra effort if it is not how they initially store them.

The current behavior is also a bit counter-intuitive as the flag can be specified multiple times but will only use the value that was provided last.