Improve messaging for rover subgraph check

[david-castaneda]

Motivation

The current message outputted by rover when running a subgraph check only tells you if the API schema changed not the core schema.

This can sometimes lead to a confusing experience. For example when adding metadata directives such as @key, @tag etc. which get removed during composition and then seeing that rover detected zero changes.

The proposed improvement is the following.

If the Core schema changed but the API schema did not change:

There were no changes detected in the composed API schema, but the core schema was modified.

If the Core and API schema did not change:

There were no changes detected in the composed schema.

[EverlastingBugstopper]

this looks great! thanks for submitting this @david-castaneda