Expand render datasource functionality

[minnerbe]

Hi, I'm a software engineer at HHMI/Janelia working on Render. Neuroglancer is an integral part of our EM-reconstruction pipeline and this PR updates the Render datasource to better align with the current capabilities of the Render web service. The key changes are:

1. Improved support for optional query parameters specified in the datasource url in the format ?key=value, which are now forwarded directly to the Render web service rather than being hardcoded in neuroglancer. This allows support for parameters introduced in recent versions or forks, without any changes to the existing url syntax.
2. Autocompletion for query parameters was added by dynamically fetching the Swagger schema of the Render service. This improves discoverability and ease of use; see screenshot below.
3. PNG support for both 8bit and 16bit sources was added. Previously, only 8bit jpeg and 16bit raw formats were supported. As we're increasingly handling 16bit sources internally, this was one of the main motivations for creating this PR.

Looking forward to your feedback!

[google-cla]

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[minnerbe]

Just to add a bit of context: I don't have a lot of experience with JavaScript/TypeScript—my main languages are Python and Java. I relied on internal feedback and some AI-based tooling to resolve language-specific issues, so I'm definitely open to suggestions on best practices or cleaner approaches!

[fcollman]

Thank you for working on this!

[minnerbe]

Thanks for your prompt response! I had to sort out the proper way to sign the CLA on behalf of HHMI/Janelia. Everything should be in order now, the corresponding check is passing.