feat: add `--field` option to `field add content-relationship`

[angeloashmore]

Summary

Content relationships should support specifying which fields to fetch from the related document via a repeatable --field flag.

Currently field add content-relationship only supports --custom-type and --tag to filter which documents can be selected, but there is no way to specify which fields to fetch from the related document.

This is separate from the help text improvement in #98.

Raised in RFC: Prismic CLI Modeling.

[angeloashmore]

For more details, see https://prismic.io/docs/fields/content-relationship