plugins::onLoad dataurl should support mimetype and base64 encoder

TLDR
1. need ability to specify **mimetype** when using `loader:dataurl` in result of the plugin `onLoad`
2. need ability to specify if dataurl will be encoded with **base64**

```js
build.onLoad({ filter: /.*/, namespace: "dataurl" }, async args => {
const buffer = await fs.readFile(args.path.replace(/\?dataurl$/, ""));
return {
contents: bufferBuffer,
loader: "dataurl"
}
};
```

the build output will be either
```
data:application/octet-stream,...
```
or this if the buffer does not have "binary" data
```
data:text/plain;charset=utf-8,...
```

now if I want to explicitly indicate the mimetype maybe for a `image/gif` or `image/jpeg`
I can not indicate that for the loader.

if `contents` also accepted `Blob`, or blob like object we can preserve the mime type
```js
let contents = new Blob([dataURLBuffer], { type: "image/gif" });
```

also need a way to indicate whether I want to use base64 encoding

```js
build.onLoad({ filter: /.*/, namespace: "dataurl" }, async args => {
const buffer = await fs.readFile(args.path.replace(/\?dataurl$/, ""));
let bufferBlob= new Blob([buffer], {type:"image/gif"})
return {
contents: bufferBlob,
base64: true, // not sure how to best communicate this
loader: "dataurl"
}
};
```

one downside with Blob, might be that it may copy the buffer
so maybe some struct support instead.

```js
build.onLoad({ filter: /.*/, namespace: "dataurl" }, async args => {
const buffer = await fs.readFile(args.path.replace(/\?dataurl$/, ""));
const struct = { buffer, mime:"image/gif", base64:true}
return {
contents: struct ,
loader: "dataurl"
}
};
```

Begin with the plugin onLoad result and loader:dataurl behavior described in the issue. Determine how the API can expose an explicit MIME type and base64 choice, then verify that generated data URLs for binary and text contents reflect those settings.

Written by the indexing model from the issue text.