feat: modify Downloader config through Blobs builder

[arilotter]

Description

A fix for #74 , allowing users of the Blobs protocol to once again provide configuration for the internally-used Downloader struct - both concurrency and retry options.
This PR exposes them through the Blobs Builder struct.

Notes & open questions

I haven't added tests, but this seems to work fine. I probably need tests for these methods to be a good OSS citizen ;)

Change checklist

• Self-review.
• Documentation updates following the style guide, if relevant.
• Tests if relevant.
• All breaking changes documented.

[rklaehn]

Thanks for the PR.

I will merge this and then deal with testability in a subsequent PR. It could be that things will get shuffled around a bit to allow testability, but the functionality to configure the downloader will be in the next release.