Extend Backend interface with CopyObject method

[ChrisSchinnerl]

Drawing some inspiration from the https://github.com/Mikubill/gofakes3 fork used by Rclone, this PR adds Copy support to this fork to avoid downloading and re uploading an object on copy.

[codecov-commenter]

Codecov Report

Merging #84 (dffef5b) into master (42c772b) will increase coverage by 0.12%. The diff coverage is 65.21%.

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@@            Coverage Diff             @@
##           master      #84      +/-   ##
==========================================
+ Coverage   65.93%   66.05%   +0.12%     
==========================================
  Files          28       28              
  Lines        2677     2687      +10     
==========================================
+ Hits         1765     1775      +10     
- Misses        647      648       +1     
+ Partials      265      264       -1     

Files Changed	Coverage Δ
backend/s3afero/multi.go	38.56% <0.00%> (-0.27%)	:arrow_down:
backend/s3afero/single.go	37.10% <0.00%> (-0.30%)	:arrow_down:
backend.go	82.85% <66.66%> (-8.45%)	:arrow_down:
backend/s3mem/backend.go	68.45% <100.00%> (+0.21%)	:arrow_up:
gofakes3.go	65.64% <100.00%> (+0.84%)	:arrow_up:

[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication