feat(): add plugin, implement the isolated schema generator

[kamilmysliwiec]

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: N/A

What is the new behavior?

• Plugin which automatically adds @Field() decorators with the metadata read from the AST
• Isolated code-first approach - we've decided to reimplement all the features by staying backward-compatible

Does this PR introduce a breaking change?

[x] Yes
[ ] No

Other information

[MarkPieszak]

🐈