feat(@schematics/angular): generate functional interceptors

[cexbrayat]

PR Checklist

Please check to confirm your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/angular/angular-cli/blob/main/CONTRIBUTING.md#-commit-message-guidelines
• [x] Tests for the changes have been added (for bug fixes / features)
• [x] 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
• [ ] Documentation content changes
• [ ] Other... Please describe:

What is the current behavior?

ng g interceptor generates a class-based interceptor

What is the new behavior?

Adds a --functional flag to the interceptor schematic to generate a functional interceptor (as we can now do for guards and resolvers)

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information