Support generation of downloadable artifacts

[marcusschiesser]

Currently, all artifacts (e.g. HTML files or code) are inline.
We should add a feature to show a generated artifact as an icon, and the user can download the artifact.
This would also allow to generate artifacts, that are not directly generated by the LLM, e.g., a PPT or PDF file.

Possible use cases:

• Write a blog post about 'xxx' -> Generates a HTML file
• Write a report about Tesla's financial situation -> Generate a PDF file.

[marcusschiesser]

#290 will add showing generated code as an artifact that could be viewed or executed.

In this ticket, we will focus on generating a PDF file and making it downloadable to the user (see https://x.com/jerryjliu0/status/1838014425120612806)

[marcusschiesser]

Assumption: The generated artifact will be served by the file server (similar to E2B artifacts).

Sub-Tasks

• @marcusschiesser Show generated PDF as an artifact and be able to download it in the UI
• @leehuwuj Extend multi-agent template to add a report-generator agent