[CIS-661] Simplify attachment send API

[mantergo]

In this PR:

• Simplify attachment send API

[codecov]

Codecov Report

Merging #815 (7f6b8ca) into main (389a317) will decrease coverage by 0.85%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##             main     #815      +/-   ##
==========================================
- Coverage   87.55%   86.70%   -0.86%     
==========================================
  Files         230      228       -2     
  Lines        8826     8636     -190     
==========================================
- Hits         7728     7488     -240     
- Misses       1098     1148      +50     

Impacted Files	Coverage Δ
...trollers/ChannelController/ChannelController.swift	91.21% <ø> (-0.03%)	⬇️
...trollers/MessageController/MessageController.swift	95.18% <ø> (-0.03%)	⬇️
Sources/StreamChat/Database/DatabaseSession.swift	92.85% <ø> (-0.17%)	⬇️
...Models/Attachments/ChatMessageAttachmentSeed.swift	95.00% <0.00%> (-5.00%)	⬇️
Sources/StreamChat/Workers/ChannelUpdater.swift	100.00% <ø> (ø)
Sources/StreamChat/Workers/MessageUpdater.swift	100.00% <ø> (ø)
Sources/StreamChat/Database/DTOs/MessageDTO.swift	98.60% <100.00%> (-0.12%)	⬇️
Tests/Shared/JSONEncoder+Extensions.swift	50.00% <0.00%> (-50.00%)	⬇️
Tests/Shared/iOS13TestCase.swift	66.66% <0.00%> (-33.34%)	⬇️
...ocketClient/Engine/StarscreamWebSocketEngine.swift	0.00% <0.00%> (-30.96%)	⬇️
... and 12 more

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[b-onc]

IMHO it looks much nicer now 🎉
Pro: Simplified API, unified flow
Con: Docs are basically required to understand these things, but seeds were not self-explanatory anyway

[VojtaStavik]

Good idea @mantergo and @b-onc 👍

[VojtaStavik]

This is an interesting side effect. I'm thinking - what does this mean? Maybe we're missing something?

[mantergo]

Encodable is the main constraint for the custom attachments cause it is the minimum we need to blindly send it to the backend.
Dummy conformance means that we can't blindly send it, some pre-processing is required and we know how to handle that.
I think that's acceptable trade-off.