115: upload files in bill

[zupzup]

This fixes #115 and adds the functionality for uploading files when issueing a bill, as well as to locally view these uploaded files.

Notes

• This changes the bill file format, so any previously saved bill will be invalid from merging this onward and will likely panic the app
• There is an issue when trying to send files, which are above a certain size (~50kb, which is around ~200kb doubly encrypted) between p2p clients using libp2p. I created an extra task for it (https://github.com/BitcreditProtocol/E-Bill/issues/218), which we might not do, since it might be a time sink and moving to Nostr fixes the issue anyway.

Besides that, I restructured a few things and added tests where possible and transitioned the contacts API to use tokio::fs, as to not block on the async runtime.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 53.06799% with 566 lines in your changes missing coverage. Please review.

Project coverage is 14.44%. Comparing base (be86fb9) to head (114defe). Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/dht/client.rs	1.23%	160 Missing :warning:
src/web/handlers/bill.rs	0.00%	135 Missing :warning:
src/persistence/bill.rs	1.44%	68 Missing :warning:
src/util/file.rs	56.25%	42 Missing :warning:
src/service/bill_service.rs	93.89%	28 Missing :warning:
src/service/mod.rs	0.00%	24 Missing :warning:
src/persistence/contact.rs	0.00%	20 Missing :warning:
src/blockchain/chain.rs	0.00%	19 Missing :warning:
src/dht/mod.rs	0.00%	14 Missing :warning:
src/main.rs	0.00%	12 Missing :warning:
... and 10 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #209 +/- ## ========================================== + Coverage 6.68% 14.44% +7.75% ========================================== Files 42 45 +3 Lines 8277 8871 +594 ========================================== + Hits 553 1281 +728 + Misses 7724 7590 -134 ```

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[mtbitcr]

LGTM

[zupzup]

Wow this one is quite massive. I can not really follow how the file upload stuff works just from the diffs but impl. looks fine. The rest of the refactoring also looks very good (as already mentioned). A bit small chunks might be easier to chew 😁

Yes, you're absolutely right, this escalated a bit :see_no_evil: - sorry! And in retrospect it seems a lot clearer, where there might have been good lines to split this by and I'll take care to keep things smaller, also for my own sanity's sake. :sweat_smile:

This will become easier with more structure as well, as it's already easier to spot which parts of the system will have to be touched by a certain change with the changes we already did. :+1: