pipedv1: Mask the plugins config in a piped config

[t-kikuc]

What would you like to be added:

Mask some fields in plugins in a piped config.

We need a discussion on how to realize this feature.

Why is this needed:

In pipedv0, some fields in the piped config were masked to hide secret values on the Web UI.

However, in pipedv1, piped core does not know which fields should be masked.

https://github.com/pipe-cd/pipecd/blob/637fc34aff646c80d2d14a4a8323b7e61a3ad27d/pkg/configv1/piped.go#L141-L156