Custom "ignore" option for fields you might not want

[chrusty]

The introduces a custom proto fieldOption. I'm not a huge fan of these because they involve magic numers from a global pool, so the only safe way to do this will be through the use of another feature flag.

• [x] options.proto which includes the new "ignore" option
• [x] Sample proto (HiddenFields.proto)
• [x] Logic to detect this option (in types.go)
• [x] Unit tests
• [x] Feature flag to enable this behaviour