Implement Kopia core flag handling data structures and flag testing suite

[plar]

Change Overview

This PR is the first in the series of PRs that will add a new way to build Kopia CLI commands.
The functionality of this PR provides the foundation for subsequent PRs and introduces a implementation of Kopia parameters handling in the CLI using the safecli package.

The Kopia flags can be represented in three ways:

• As an argument, i.e., /restore/dir or 01234566789abcdef.
• As a string flag, i.e., --flag=value, where the value can be a sensitive field for log output.
• As a boolean/switch flag, i.e., --readonly, where the flag is present without any value.

To support this behavior, this PR introduces two primary data structures:

• The pkg/kopia/cli/internal/flag/stringFlag and pkg/kopia/cli/internal/flag/boolFlag structs provide a structured approach to managing flags, encapsulating their properties and behaviors.
• The redacted flag handling in stringFlag ensures that sensitive information is managed safely.

Functions such as pkg/kopia/cli/internal/flag/NewStringFlag, pkg/kopia/cli/internal/flag/NewBoolFlag, etc will be utilized in upcoming PRs for creating Kopia CLI commands.

pkg/kopia/cli/internal/flag/test/FlagTest struct and related methods provide a framework for defining and executing flag tests (see pkg/kopia/cli/internal/flag/flag_test.go).

pkg/kopia/cli/errors.go will serve as a centralized location for error definitions.

Looking forward for any feedback/suggestions!

Pull request type

Please check the type of change your PR introduces:

• 🚧 Work in Progress
• 🌈 Refactoring (no functional changes, no api changes)
• 🐹 Trivial/Minor
• 🐛 Bugfix
• 🌻 Feature
• 🗺️ Documentation
• 🤖 Test

Test Plan

• 💪 Manual
• ⚡ Unit test
• 💚 E2E