Adds support for private Backblaze buckets

[allancalix]

Using the client that is already used by B2 storage to make the Get call, this PR adds support for reading to/from both private and public buckets. Incidentally, this also makes setting the b2-url field in the config unnecessary.

Note that removing the B2Url field from the config struct is a breaking change so I left it as is, I'm happy to do something different according to the project's update policy.

[Bren2010]

I believe this was actually intentional because Backblaze will charge you for egress if you do it this way. But if you set your bucket up to be accessed through Cloudflare, then it's free because of Bandwidth Alliance.

[allancalix]

I see, it looks like it is possible to support this using Cloudflare workers with private buckets. In this change's current state, I'm happy to close this PR if it's not something the project wants to support or make the following changes:

1. Update docs to note egress savings via Cloudflare CDN
2. Provide a configuration option to override the Backblaze domain to continue to support the use case
3. Otherwise, use the client for easier setup / private bucket support

[Bren2010]

Yes, I think that would be good. You can leave the documentation changes out though and I'll add that in another PR since I've done it before

[allancalix]

SGTM, updated the PR. This should should essentially be noop for existing configurations but support private buckets / api-based public downloads.

[Bren2010]

Awesome, thanks so much! I'll write the docs I said I would tonight