feat(gatsby-source-medium): define types explicitly via createSchemaCustomization (#22377)

* feat(gatsby-source-medium): add types via createSchemaCustomization

* fix(gatsby-source-medium): declare correct type for date fields

Some of them were incorrectly defined as Int.

* refactor(gatsby-source-medium): readability improvements

* chore: format

* fix: apply review suggestions

Co-authored-by: gatsbybot <mathews.kyle+gatsbybot@gmail.com>

Author: Powell-v2