[Feature Request]: Support for source maps without release_id

[TrevorBurnham]

Please provide your feature request

I was excited to see Cloudwatch RUM get support for unminification with source maps: #231 However, looking at the user guide for the feature, I realized that it would be challenging to set up for my application, which has no existing notion of a release_id.

For context, I'm using Next.js, which follows the popular convention across many JavaScript frameworks of:

1. Using hashes to generate a unique name for each .js chunk (based on its contents), and
2. Emitting .js and .js.map files as siblings (if enabled).

I upload all of those assets to S3, so that e.g. a request for /_next/static/chunks/f8ca1dff67f056a5.js would go to /.next/static/chunks/f8ca1dff67f056a5.js in my bucket, and its corresponding source map would be located at /.next/static/chunks/f8ca1dff67f056a5.js.map in my bucket.

What would really be convenient would be to have a way to tell RUM how to perform that mapping from .js filename to .js.map filename. I'd suggest configuration parameters along the lines of routePrefix and bucketPrefix: In my case, I'd specify routePrefix: "/_next/static/chunks" and bucketPrefix: "/.next/static/chunks". Then RUM could look up the source map for any requested .js file by subtracting the routePrefix, adding the bucketPrefix, and changing the file extension from .js to .js.map.