feat: add markdown support to banner message

[Ankurk99]

Summary

Harbor provides a global configuration for setting a banner message, which is useful for displaying important notices to users, especially for maintenance.
One of our use-cases requires adding links to these messages to direct our users to more information. However, since BannerMessage is an HTML-safe string, it does not support HTML tags, making it difficult to include links.

This PR adds support for markdown in the banner message.

Issue being fixed

Fixes #21657

Please indicate you've done the following:

• Well Written Title and Summary of the PR
• Label the PR as needed. "release-note/ignore-for-release, release-note/new-feature, release-note/update, release-note/enhancement, release-note/community, release-note/breaking-change, release-note/docs, release-note/infra, release-note/deprecation"
• Accepted the DCO. Commits without the DCO will delay acceptance.
• Made sure tests are passing and test coverage is added if needed.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed in website repository.

[Ankurk99]

Please let me know if any updates are needed for test coverage or documentation.

[xuelichao]

LGTM

[xuelichao]

Please fix the pipeline error