Add Introspection for mutation input hook.

[id3nom]

Description

Add pgIntrospection and pgFieldIntrospection in context.scope for mutation input hook.

https://github.com/graphile/graphile-engine/issues/742

Performance impact

unknown

Security impact

unknown

Checklist

• [x] My code matches the project's code style and yarn lint:fix passes.
• [ ] I've added tests for the new feature, and yarn test passes.
• [ ] I have detailed the new feature in the relevant documentation.
• [ ] I have added this feature to 'Pending' in the RELEASE_NOTES.md file (if one exists).
• [ ] If this is a breaking change I've explained why.

[benjie]

What's the motivation for this?