Adds support for [Federation specification
v2.5](https://www.apollographql.com/docs/federation/federation-versions#v25).

Changes:

* new `@authenticated` directive

```graphql
directive @authenticated on
    ENUM
  | FIELD_DEFINITION
  | INTERFACE
  | OBJECT
  | SCALAR
```

* new `@requiresScopes` directive

```graphql
directive @requiresScopes(scopes: [[Scope!]!]!) on
    ENUM
  | FIELD_DEFINITION
  | INTERFACE
  | OBJECT
  | SCALAR

scalar Scope
```

)

Attempting to import new directives with old specification version will
throw exception.

Add missing `Purpose` enum. Full `@link` definition as per the
[spec](https://github.com/apollographql/specs/blob/main/link/v1.0/link-v1.0.graphql):

```graphql
directive @link(
    url: String!,
    as: String,
    import: [Import],
    for: Purpose)
repeatable on SCHEMA

scalar Import

enum Purpose {
  SECURITY
  EXECUTION
}
```

Resolves: #351

Federation tracing should only be enabled if the Router/Gateway sends
the proper `ftv1` header and value. Currently trace data is generated
even if the header is not specified which leads to unnecessary
computations of data that will ultimately be discarded by the
Router/Gateway.... If header is not specified, we should default to
`false` to avoid those unnecessary computations.

See: https://www.apollographql.com/docs/federation/metrics/

Supersedes: #368

* federation v2.6 - new `@policy` directive, see
[docs](https://www.apollographql.com/docs/federation/federated-types/federated-directives#policy)
for details

```graphql
directive @Policy(policies: [[federation__Policy!]!]!) on
  | FIELD_DEFINITION
  | OBJECT
  | INTERFACE
  | SCALAR
  | ENUM

scalar Policy
```

* federation v2.7 - update to `@override` definition (new `label`
argument), see
[docs](https://www.apollographql.com/docs/federation/federated-types/federated-directives#progressive-override)
for details

```graphql
directive @OverRide(from: String!, label: String) on FIELD_DEFINITION
```

---------

Co-authored-by: Trevor Scheer <[email protected]>

Starting with Federation v2.3 `@tag` is applicable on `SCHEMA` as well.

Fixes #380

This restructures the scalar transformation logic to ensure that both
the type definition and wiring are added correctly if necessary. This
allows consumers to define the scalars in their schema, but not assign
runtime wiring. This can be nice to support other tools like linters or
graphql-faker that require the schema to be a valid schema file.