feat: add slogext package and switch to use it

[jaredallard]

This commit adds a slogext package with the purpose of providing a generic logging package, powered by slog^1, with charm/log^2 as the underlying handler. This enables us to have an easier time switching logging libraries out if we ever need to do so again.

I also added the notion of a test logger that will enable us to do test specific things if we so desire (e.g., pipe to t.Log, automatic test.name setting, etc.). I switched all tests to use this.

Misc changes made:

• Added a Apache-2.0 license header to the snippets replacing the stable AGPL one there previously.
• Fixed a bug in the stenciltest error handling that would cause expecting an error to never actually work.

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 58.20896% with 28 lines in your changes are missing coverage. Please review.	Files	Patch %	Lines
internal/codegen/tpl_stencil.go	28.57%	10 Missing :warning:
internal/slogext/slogext.go	76.47%	4 Missing :warning:
cmd/stencil/stencil.go	0.00%	3 Missing :warning:
internal/codegen/stencil.go	60.00%	2 Missing :warning:
pkg/extensions/apiv1/server.go	0.00%	2 Missing :warning:
pkg/stenciltest/stenciltest.go	50.00%	1 Missing and 1 partial :warning:
internal/cmd/stencil/stencil.go	50.00%	1 Missing :warning:
internal/codegen/tpl_file.go	0.00%	1 Missing :warning:
internal/modules/module.go	0.00%	1 Missing :warning:
pkg/extensions/apiv1/rpc_transport_server.go	0.00%	1 Missing :warning:
... and 1 more

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