Add MultipartBackend interface to support overriding the multipart upload behaviour

[ChrisSchinnerl]

This PR adds support for implementation the optional MultipartBackend interface to avoid the in-memory multipart uploader and instead use a custom one.

I have to admit I accidentally opened this PR and the Copy PR on this repo instead of our fork but I thought it might be a good addition anyway.

[codecov-commenter]

Codecov Report

Merging #83 (737ad23) into master (42c772b) will decrease coverage by 0.14%. The diff coverage is 62.33%.

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@@            Coverage Diff             @@
##           master      #83      +/-   ##
==========================================
- Coverage   65.93%   65.79%   -0.14%     
==========================================
  Files          28       28              
  Lines        2677     2681       +4     
==========================================
- Hits         1765     1764       -1     
- Misses        647      649       +2     
- Partials      265      268       +3     

Files Changed	Coverage Δ
backend.go	91.30% <ø> (ø)
gofakes3.go	65.15% <46.66%> (+0.35%)	:arrow_up:
uploader.go	84.61% <66.12%> (-3.91%)	:arrow_down:

[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
1 Code Smell

No Coverage information
0.0% Duplication

[shabbyrobe]

Looks great! Thank you!