✨ Add TemporaryEnvironmentVariable test fixture (to be used in a managed identity connection, for example)

[stijnmoreels]

Is your feature request related to a problem? Please describe.
I noticed that I had to implement the TemporaryEnvironmentVariable multiple times now for projects that need either managed identity connection or when the test locally starts up the app and sets certain variables.

Describe the solution you'd like
A test fixture called TemporaryEnvironmentVariable that temporary sets an environment variable during the lifetime of the fixture.
Should be in the Arcus.Testing.Core library.

[stijnmoreels]

Closely related to this is a possible TemporaryMangedIdentityConnection test fixture that would build upon these temporary environment variables to temporary set a service principal managed identity connection to authenticate more easily with any additional Azure resource during the test run.