feat: add concurrency support for pull and push operations

[chlins]

This pull request introduces several enhancements and refactorings to improve the concurrency and configuration handling of the pull and push commands. The most important changes include the addition of concurrency options, validation for configuration, and refactoring of backend options.

Enhancements to Concurrency:

• Added a new concurrency option to the pull and push commands, allowing users to specify the number of concurrent operations. (cmd/pull.go, cmd/push.go, pkg/config/pull.go, pkg/config/push.go, pkg/backend/options.go) [1] [2] [3] [4] [5]

Configuration Validation:

• Introduced validation for the pull and push configurations to ensure the concurrency value is valid. (cmd/pull.go, cmd/push.go, pkg/config/pull.go, pkg/config/push.go) [1] [2] [3] F52d5529L25R33)

Refactoring of Backend Options:

• Refactored the backend options to use a more streamlined approach for setting options, including the new concurrency option. (pkg/backend/options.go, cmd/login.go, cmd/pull.go, cmd/push.go) [1] [2] [3] [4]

Parallelization of Layer Copy:

• Implemented parallelization of layer copy operations in the pull and push backend methods using errgroup. (pkg/backend/pull.go, pkg/backend/push.go) [1] [2]

Dependency Addition:

• Added the errgroup package to handle concurrent operations in the backend. (pkg/backend/pull.go, pkg/backend/push.go) [1] [2]

[gaius-qi]

LGTM