Add public API analysers

[martincostello]

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

Examples:

• https://github.com/domaindrivendev/Swashbuckle.AspNetCore/pull/2418#pullrequestreview-2000191284.
• https://github.com/domaindrivendev/Swashbuckle.AspNetCore/pull/2610#discussion_r1565295657
• https://github.com/domaindrivendev/Swashbuckle.AspNetCore/pull/2677#discussion_r1565538897
• https://github.com/domaindrivendev/Swashbuckle.AspNetCore/pull/2718#discussion_r1566883950
• https://github.com/domaindrivendev/Swashbuckle.AspNetCore/pull/2803#discussion_r1566871645

[github-actions[bot]]

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.