feat(utils): treat env as plain string and default to "dev"

[TordAreStromsnes]

We remove the Environment enum from the configuration codebase and updates all related logic, documentation, and tests to treat the env parameter as a plain string with a default value of "dev". The changes simplify environment handling.

Core codebase changes:

• Removed the Environment enum and all references to it from the codebase, including imports, type annotations, and default values. The env parameter is now a plain string throughout models and logic, defaulting to "dev" when not set.

• Updated the PipelineParameterManager.prepare_infrastructure method to no longer require env to be set in the environment; it now defaults to "dev" if missing and returns env as a plain string in the InfraContext.

Documentation updates:

• Revised documentation to reflect that env is now optional, defaults to "dev", and is a plain string (not an enum). Clarified lookup strategy and migration notes for users.

Test and usage updates:

• Updated all tests and example usages to set or expect env as a plain string instead of using the removed enum.

Validation and printing improvements:

• Simplified configuration printing to directly show the string value of env.