Refactor/mocks sub package

[mbruzina]

This is a draft for introducing some different testing methodologies in an effort to increase test coverage and refactor some of our existing logic.

Starting with /internal/install/discovery, a new mocks directory was introduced, and mocks were built from Discoverer and Validator interfaces using mockery. The end result within the tests appears similar to the previous stubs, but the package is a bit cleaner overall.

Some of the existing mocks in this package seemed a bit odd in how they were used; this led to some refactoring of logic within recipe_installer and focused on removing receiver objects from RecipeInstall that don't really need to be there, such as os discover and manifest validation. These two receivers were extracted into function variables which provides the flexibility to override their implementation in recipe_installer_test as well as remove unnecessary members from RecipeInstall

[codecov]

Codecov Report

Merging #1393 (cfead4c) into main (8a124b1) will increase coverage by 0.32%.
The diff coverage is 97.61%.

❗ Current head cfead4c differs from pull request most recent head 9534cea. Consider uploading reports for the commit 9534cea to get more accurate results

@@            Coverage Diff             @@
##             main    #1393      +/-   ##
==========================================
+ Coverage   47.01%   47.33%   +0.32%     
==========================================
  Files         119      111       -8     
  Lines        7079     6889     -190     
==========================================
- Hits         3328     3261      -67     
+ Misses       3616     3493     -123     
  Partials      135      135              

Impacted Files	Coverage Δ
internal/install/mock_recipe_installer.go	16.66% <ø> (+4.16%)	⬆️
internal/install/recipe_installer.go	79.13% <92.30%> (+0.53%)	⬆️
...al/install/discovery/manifest_discovery_service.go	100.00% <100.00%> (ø)
internal/install/discovery/manifest_validator.go	92.50% <100.00%> (ø)
internal/install/license_key_fetcher.go	100.00% <100.00%> (+100.00%)	⬆️
internal/install/recipe_install_builder.go	90.00% <100.00%> (-0.91%)	⬇️
internal/install/validation/agent_validator.go	76.66% <100.00%> (ø)
internal/install/mocks/mock_context.go
...l/install/validation/mocks/mock_agent_validator.go
...ernal/install/validation/mocks/mock_nrdb_client.go
... and 3 more

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