Manage models needs to expose additional providers

[isidorn]

Right now the model picker shows a fixed set of models. With the finalization of #250007 we need to have a nice UI entry point for users to discover more providers.

Low effort:

separator
Install Language Model Provider Extension...
opens Extensions view with tag:"language-model" filtered down. Ref microsoft/vscode-vsce#1165

Med effort (lovely idea by @pierceboggan):
separator
we list all the provider names from all extensions on the Marketplace

We can get all the provider names from the vscode-unpkg service, similar like @aeschli does for themes. @sandy081 can confirm what queries he supports for us to get all extensions with "languageModels" in package.json contribution

[isidorn]

Code pointer from @aeschli

vscode/src/vs/workbench/contrib/themes/browser/themes.contribution.ts

Line 115 in 0fd5ef7

const pager = await this.extensionGalleryService.query(options, token);

So the query goes to marketplace, searches for tag:icon-theme in some cases, and then goes to unpkg service for each extension to get the package.json content. We should do the same to show a full list of language model providers.

I understand it might slow down the Manage Models.. experience. Ideally, we update the quick pick async as the results come back.

[isidorn]

Discussed with Ben, and we have picks and additionalPicks API. So picks we would fill with built-in providers, and additionalPicks are coming in async which would be the result from the MP query.

[isidorn]

I know Logan is busy in June. And @pierceboggan and me want to do some vibe-coding in June. So assigning to the two of us to try to do this.

@lramos15 let me know if you have concerns.

[lramos15]

I don't really know if any work can be done here prior to API changes

[isidorn]

@lramos15 we can give proposed API access to some early adopters if we so decide.
Also having the UI sketched out will help those extension authors figure out how the experience will look like and why we want to move most providers to an extension.