Install NuGet provider when missing

[kingsleyck]

Pull Request (PR) description

Automatically install the NuGet package provider where required. This feature was added to prevent an interactive prompt to install the provider which will cause DSC application to hang.

This Pull Request (PR) fixes the following issues

None

Task list

• [ ] Added an entry under the Unreleased section of the change log in the CHANGELOG.md. Entry should say what was changed, and how that affects users (if applicable).
• [ ] Resource documentation added/updated in README.md.
• [ ] Resource parameter descriptions added/updated in README.md, schema.mof and comment-based help.
• [ ] Comment-based help added/updated.
• [ ] Localization strings added/updated in all localization files as appropriate.
• [ ] Examples appropriately added/updated.
• [x ] Unit tests added/updated. See DSC Resource Testing Guidelines.
• [ ] Integration tests added/updated (where possible). See DSC Resource Testing Guidelines.
• [x] New/changed code adheres to DSC Resource Style Guidelines and Best Practices.

[codecov-io]

Codecov Report

Merging #4 into master will increase coverage by 0.28%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #4      +/-   ##
==========================================
+ Coverage   94.49%   94.78%   +0.28%     
==========================================
  Files           2        2              
  Lines         109      115       +6     
==========================================
+ Hits          103      109       +6     
  Misses          6        6

Impacted Files	Coverage Δ
...ces/PowershellRepository/PowershellRepository.psm1	90.76% <100%> (+0.44%)	:arrow_up:
...ssResources/PowershellModule/PowershellModule.psm1	100% <100%> (ø)	:arrow_up:

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