feat(config): allow for flat generate option configuration

[adamclark64]

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?

currently a user needs to pass a --flat flag every time desired when running a generate command

Issue Number: closes #1518

What is the new behavior?

allows for flat to be set in generateOptions configuration. defaults to false

{
  "generateOptions": {
    "flat": true
  }
}

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[micalevisk]

after merging this PR could you please update the JSON schema at https://github.com/SchemaStore/schemastore as well? you will need to touch the src/schemas/json/nest-cli.json and src/test/nest-cli/nest-cli.json files 😺

[adamclark64]

can update here once this is merged as well

[kamilmysliwiec]

lgtm