Add implementation code generation

[minhaj10p]

Feature: Code generation for Implementation through Excel sheet and input profile.
Uses package https://github.com/satori/go.uuid to generate GUIDs
Usage: oscalkit implementation -p path-to-profile -e path-to-excelsheet.csv

[codecov]

Codecov Report

Merging #32 into master will increase coverage by 13.01%.
The diff coverage is 92.81%.

@@             Coverage Diff             @@
##           master      #32       +/-   ##
===========================================
+ Coverage   65.62%   78.64%   +13.01%     
===========================================
  Files           3        4        +1     
  Lines         160      309      +149     
===========================================
+ Hits          105      243      +138     
- Misses         45       52        +7     
- Partials       10       14        +4

Impacted Files	Coverage Δ
generator/mapper.go	81.15% <100%> (ø)	⬆️
generator/reader.go	58.82% <60%> (-2.41%)	⬇️
impl/implementation.go	93.87% <93.87%> (ø)

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[mohuk]

@anweiss @minhaj10p Just a comment. I feel this code generation of implementation.go shouldn't be part of OSCALkit since the implementation generated is specific to Docker. I am sure we are going to move this out of OSCALkit down the road.

[anweiss]

LGTM

[anweiss]

LGTM

[anweiss]

@mohuk per your comment, can you create an issue for us to address the Docker-specific code that was merged in this PR at a later point?