feat: introduce fetch command to get partial files

[chlins]

This pull request introduces a new fetch command to the modctl command-line tool, allowing users to fetch partial files from a remote repository. The most important changes include adding the fetch command implementation, updating the backend interface, and configuring the fetch operation.

New fetch command implementation:

• cmd/fetch.go: Added a new fetch command with various flags for configuration, including concurrency, plain HTTP, insecure connection, proxy, output directory, and patterns. The command validates the configuration and runs the fetch operation.
• cmd/root.go: Registered the new fetch command with the root command.

Backend updates:

• pkg/backend/backend.go: Added a Fetch method to the Backend interface to support fetching partial files.
• pkg/backend/fetch.go: Implemented the Fetch method, which handles the fetching of partial files based on specified patterns, using a remote repository and supporting various configurations such as proxy and concurrency.

Configuration and testing:

• pkg/config/fetch.go: Added a new Fetch configuration struct with validation methods and default values.
• pkg/backend/fetch_test.go: Added tests for the Fetch method to ensure it correctly handles various scenarios, including pattern matching and invalid references.

Linter configuration:

• .golangci.yml: Updated the linter configuration to include a new prefix for the github.com/CloudNativeAI/modctl package and excluded the test/mocks directory from linting. [1] [2]
• .golangci.yml: Modified the output format configuration to support multiple formats.

[gaius-qi]

LGTM

[gaius-qi]

LGTM