Support info banner populated from a data source

### Description

A deployed ConstructHub is a website with potentially anonymous visitors (if deployed publicly). As such there is no direct way to communicate updates, news or status to the visitors.

ConstructHub should support displaying messages (at least a single short message containing a link to further information) prominently on the site. This message should be populated from a configurable source (like a uri to a json file or feed) and be updatable without the need to re-deploy the system. For example the uri could be retrieved periodically in the background and the message written to a cache that is read from when the webpage is loaded.

Likely requires a change to https://github.com/cdklabs/construct-hub-webapp as well.

### Use Case

- Provide a status to an ongoing issue
- Highlight news are announcements
- Note about planned downtime

### Other information

_No response_