feat: set fetch options

[b3nten]

Certain packages export different contents depending on the target platform for compilation. For example, there is a significant difference bundling @solid-js/web for the server versus the client, eg. https://esm.sh/stable/[email protected]/deno/web.js & https://esm.sh/stable/[email protected]/es2022/web.js.

By allowing users to set the user-agent header we can bundle for different targets besides deno. I'm think there could also be other usecases for this, such as bundling authenticated resources etc.

I've gone ahead and passed an RequestInit object from the plugin options to the portable loader. It doesn't seem like it's possible to change how the native loader fetches data though.