refactor(graphql/lib): definitions generator defaultTypeMapping type reinforcement

[Neosoulink]

PR Checklist

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Issue Number: #3210

What is the new behavior?

Reinforce the DefinitionsGeneratorOptions.defaultTypeMapping interface for better visual over the allowed types.

Better suggestions over the supported types

Does this PR introduce a breaking change?

• Yes
• No

Other information

will close #3210

[kamilmysliwiec]

LGTM