Usage reporting plugin needs to support input fields

Usage reporting seems very incomplete without knowing the usage of input fields.

The variable reporting is insufficient for the use case of checking apollo studio to see whether a deprecated input field is still being used, so it can be removed. This is because you can hard code values into a graphql query (not input them as variables) and the [`sendVariableValues`](https://www.apollographql.com/docs/apollo-server/api/plugin/usage-reporting/#valid-sendvariablevalues-object-signatures) will not report these values.

There is this merge request that was reverted and never reintroduced for some reason https://github.com/apollographql/apollo-server/pull/4654 (cc @sachindshinde)

Start by reviewing the usage reporting plugin and the sendVariableValues documentation, then compare the reverted pull request #4654 with the current implementation. Define the expected reporting behavior for hard-coded GraphQL input fields and verify that Apollo Studio can use the resulting data to identify deprecated fields.

Written by the indexing model from the issue text.