feat(swagger): JSON and YAML document endpoints customizable

[jcolladosp]

Same PR as #2046

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
• 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?

Right now the swagger json and yam are deployed in ${finalPath}-json and ${finalPath}-yaml, without possibility of customization:

http://localhost:3000/api-json
http://localhost:3000/api-yaml

Issue Number: #1968

What is the new behavior?

Added the options jsonDocumentUrl and yamlDocumentUrl in the interface SwaggerCustomOptions
If the option value is present, json and/or yaml documents are served in the specified url.
Furthermore, if globalPrefix is specified in the nest options, it's used as prefix for the custom urls.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[kamilmysliwiec]

LGTM