support config rules for masking fields

[cppforlife]

when showing diffs (--diff-changes) some users want to be able to mask values of Secrets (v1/Secret). let's add diffFieldMaskRules to config:

apiVersion: kapp.k14s.io/v1alpha1
kind: Config

diffFieldMaskRules:
- path: [data]
  resourceMatchers:
  - apiVersionKindMatcher:
      apiVersion: v1
      kind: Secret

[cppforlife]

added support for masking for deploy command in https://github.com/k14s/kapp/releases/tag/v0.18.0

[cppforlife]

delete cmd uses default mask rules in https://github.com/k14s/kapp/releases/tag/v0.25.0

[rohitagg2020]

Have to implement the same for other commands. https://carvel.dev/kapp/docs/latest/config/#diffmaskrules

[renuy]

Pending: delete does not mask based on previously used config (used during deploy as diffmask config is not saved) and uses the default config (which is just secrets). Option to close this issue and open an issue to address the pending task.