Add TrustedRootJSON to TrustRootSpec

[codysoyland]

This adds TrustedRootJSON to the TrustRootSpec to allow using of the TrustedRootJSON spec in air-gapped environments.

usage:

apiVersion: policy.sigstore.dev/v1alpha1
kind: TrustRoot
metadata:
  name: my-trusted-root
spec:
  trustedRootJSON: |
    {
      "mediaType": "application/vnd.dev.sigstore.trustedroot+json;version=0.1",
      "certificateAuthorities": [
        {
...

Note this uses type string. We could alternatively use type []byte for this, which would mean that the trustedRootJSON value would need to be base64-encoded. That would be perhaps less prone to human-error in indentation. I'm happy to make that change if deemed better by the maintainers.

Fixes #1704

Summary

Release Note

Documentation