Add public API analysers

[martincostello]

We should add the Microsoft.CodeAnalysis.PublicApiAnalyzers analyser to avoid making unintentional binary-breaking changes.

Examples:

• Observe the route template constraints in the Swagger middleware #2418 (review).
• Added Swashbuckle.AspNetCore.Annotations.SwaggerIgnoreAttribute #2610 (comment)
• Add customized document serialization support #2677 (comment)
• Resolves #2717 #2718 (comment)
• Configure non-nullable types as required #2803 (comment)
• Support [DataMember] IsRequired in NewtonsoftDataContractResolver #2644 (comment)

[github-actions]

This issue is stale because it has been open for 60 days with no activity. It will be automatically closed in 14 days if no further updates are made.

[martincostello]

Will do a PR for this once 6.7.0 ships.