feat(era): Add EraStream that downloads all era files

[RomanHodulak]

Closes #15496

Adds EraStream to stream filenames of downloaded files in a given folder.

Some rests:

• Add checksum validation to detect files integrity
  • When continuing download from an interrupted run, the latest file might have not been downloaded fully
  • Any other data integrity corruption
• ☑️ Tests could be rewritten so that they don't depend on real network
  • Gives more control and possibility to cover more test cases more easily and predictably
  • Increases tests performance
  • Makes tests not depent on availability of external online services
• Add sleep inbetween some futures
  • When folder is full and there are no ongoing downloads, the polls to recount files are happening very quickly
  • Adding a sleep future might avoid a bunch of unneeded polls

[mattsse]

great start, I think most of the functionality is already there.

I have some suggestions for structuring this, I think converting this into an actual stream type will be more flexible later, for example if we need to then route the downloaded files somewhere else we can easily add a channel on top

[mattsse]

this is looking great, love the tests.

have some (pedantic) nits.

I think we should also consider sprinkling a few trace! calls here and there, e.g. when we started/finished downloading a file

[mattsse]

nice, left some suggestions, but otherwise this lgtm

The polls are happening very quickly even when nothing is happening

this could be due to the waker, which, from what I can see, can be removed entirely

Add checksum validation to detect files integrity
When continuing download from an interrupted run, the latest file might have not been downloaded fully
Any other data integrity corruption

all of these additional features we def want but can be added as followups