Add integrity checksums

[tafaust]

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?

As of now, the swagger-ui has no checksums on style or script.

Issue Number: #2667

What is the new behavior?

Checksums for custom js or css can be set via module options.

Does this PR introduce a breaking change?

• Yes
• No

The SwaggerCustomOptions changed. In particular the customJs, customJsStr, customCssUrl.

Other information

Tests and conventional commit conformance will be aligned with contribution guidelines upon non-draft status.

[tafaust]

In case the oauth2-redirect.html from the dependency dist is used, please see swagger-api/swagger-ui#7540 (comment) where I hope SRI is included in the near future.

[tafaust]

May I receive some feedback on my question here #2667 (comment)?