Refactor env provider into a major breaking version /v2 (#341)

- `providers/env` is now `/providers/env/v2`
- Add new `Opt{}` struct with transform functions that can be passed to the
  constructor instead of the old `WithProvider(..)` functions. This allow future
  callbacks and options to be added without breaking changes.
- `Opt.TransformFunc` is now a single transformation function for keys and values.
- `Opt.EnvirnFunc` allows the source of environment variables to be plugged
  for testing and mocking. It defaults to `os.Environ`.

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Co-authored-by: Kailash Nadh <kailash@nadh.in>

Replace yaml package with go.yaml.in/yaml/v3 (#365)

* Replace yaml package with github.com/goccy/go-yaml

* Replace yaml package with go.yaml.in/yaml/v3

Tidy go.mod

Add `Unflatten` config option to nats provider (#355)

Add `urfave/cli/v3` provider (#361)

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Co-authored-by: Gabriel P. <gabriel.pt@planethoster.info>

Add new init method with config to cliflagv2 to set default fields.

Update min Go version in examples and go.work.

Update min Go version in examples and go.work.