feat(@nestjs/graphql): add enumsAsTypes to definitions generator options

[Koala-gentil]

Add enumsAsTypes option to generate string literal union types instead of enums.

Closes #1557

PR Checklist

Please check if your PR fulfills the following requirements:

• 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
[x] 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: #1557

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

[kamilmysliwiec]

LGTM