feat: Support for config file reading

[jellonek]

Add support for configuration files as described in #49

TODO:

• update README.md
• remove assumption for correct value of $HOME (by using https://pkg.go.dev/os#UserHomeDir instead of plain env variable)
• * add test coverage
  (* not mentioned in acceptance criteria but seems to be reasonable).

[Copilot]

Pull request overview

This PR starts adding configuration management support (per #49) by introducing Viper-based config/env initialization in the CLI entrypoint, along with a --config flag to point to an explicit config file.

Changes:

• Add Viper initialization and multi-location config file discovery logic in main.go.
• Add a --config CLI flag (currently via pflag) to specify a custom config file path.
• Promote github.com/spf13/pflag to a direct dependency in go.mod.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 8 comments.

File	Description
main.go	Adds Viper setup, config-file discovery, and a --config flag prior to running the Cobra CLI.
go.mod	Adds spf13/pflag as a direct dependency to support the new flag parsing.

Comments suppressed due to low confidence (1)

main.go:57

• Typo in comment: the listed fallback file name "$HOME/cloudclt.yaml" should be "$HOME/cloudctl.yaml".

		//   ./cloudctl.yaml
		//   $HOME/cloudclt.yaml
		//   $XDG_CONFIG_HOME/cloudctl/cloudctl.yaml

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[onuryilmaz]

Left a few small notes below, mostly docs alignment and one clarifying comment request. Nothing blocking.