Improve error messages for value types mismatches

This PR adds a fix for how we check value types: https://github.com/apollographql/federation/pull/280 but the error message could be improved, for example:

```javascript
const [definitions] = createDocumentsForServices([
{
typeDefs: gql`
type Person {
age(relative: Boolean!): Int
}
`,
name: 'serviceA',
},
{
typeDefs: gql`
type Person {
age(relative: Boolean): Int
}
`,
name: 'serviceB',
},
]);

const errors = validateSDL(definitions, schema, [
UniqueTypeNamesWithFields,
]);
```

Returns this error:

```
[serviceA] Person -> A field's input type (`relative`) was defined differently in different services. `serviceA` and `serviceB` define `relative` as a Boolean! and Boolean respectively. In order to define `Person` in multiple places, the input values and their types must be identical.
```

It would be nice to include the field name in the error, like this:

```
[serviceA] Person.age -> A field's argument (`relative`) was defined differently in different services. `serviceA` and `serviceB` define the argument `relative` on `Person.age` as a Boolean! and Boolean respectively. In order to define `Person` in multiple places, the input values and their types must be identical.
```

Link to discussion and code: https://github.com/apollographql/federation/pull/280/files#r535572567

Start with the validateSDL entry point and the UniqueTypeNamesWithFields rule described in the issue, then review PR #280 and its linked discussion for the existing value-type check. Update the mismatch error so it includes the field path and argument context shown in the expected message, and verify the example produces that wording.

Written by the indexing model from the issue text.