Extend `@astrolib/seo` API to Support Custom Tags and Metadata

[mseele]

Feature Request:

I would like to request an extension of the @astrolib/seo API to allow for custom tags and metadata, similar to the following features available in Pagefind:

1. Custom URL Tagging:
   Allow for the overriding of default URLs by supporting custom URL definitions in the metadata. Pagefind’s UI Usage documentation provides a good example of this functionality. This would be beneficial in cases where content is accessed via custom URLs or canonical links, providing more control over search result linking.

2. Support for Custom Metadata Tags:
   Expand the metadata capabilities to include additional custom tags and attributes, such as titles, descriptions, and other SEO-related tags that can be applied globally or overridden on a per-page basis. Pagefind's metadata documentation showcases the utility of customizable tags and metadata for enhanced content management.

Why This Feature is Valuable:

• Greater Flexibility for SEO Optimization: By enabling custom tags, users can better control how content is presented in search results, which can lead to improved SEO outcomes.
• Enhanced Content Management: Custom tags allow users to categorize, filter, and manage content more effectively, making @astrolib/seo more adaptable to diverse use cases.
• Better User Experience: URL overriding and customizable metadata would allow users to create more user-friendly, relevant, and accurate search results, which is especially useful for complex content structures.