feat: support project query parameter (#69)

[daveleek]

Description

This PR aims to add the projectId query parameter feature to the unleash-client-dotnet.
It does this by adding a setting for ProjectId to UnleashSettings. This setting is carried over to UnleashApiClient and there gets added to the feature toggles uri.

Fixes # (issue)
#69

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

It was tested by adding unit tests specifically for this feature and by running the entire test suite.
Also tested by debugging the WebApplication project in this repo with the different projectids from the demo host.
One unittest fails, but that also fails on master without my changes.: ShouldBeEnabledWhenExpected

• UnleashApiClient_Project_Tests.FetchToggles_ForProject
• UnleashApiClient_Project_Tests.FetchToggles_WithoutProject

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[chriswk]

Cool. Looks good to me