[flutter_appauth_platform_interface] Add Prompt class to expose all prompt defined in OpenID Connect 1.0 specs

[vmichalak]

This pull request introduces a Prompt class that exposes standardized values for the prompt parameter. The class provides constant string values that align with the OpenID Connect Core 1.0 specification.

Purpose

• Simplify developer experience when working with OpenID Connect authentication flows by providing type-safe, well-documented constants for prompt parameter values.
• Being aligned with existing Prompt class in native AppAuth library

[MaikuB]

Thanks for the PR. It's a good one to make it easier but could you adjust the API docs so they more align with the Effective Dart: Documentation guide?

Main ones that have stuck out to me

• https://dart.dev/effective-dart/documentation#do-start-doc-comments-with-a-single-sentence-summary
• https://dart.dev/effective-dart/documentation#do-separate-the-first-sentence-of-a-doc-comment-into-its-own-paragraph

[MaikuB]

I ended up aplying the changes. Thanks!