[28.x backport] cli/command: add WithUserAgent option

[thaJeztah]

backport:

• cli-plugins/plugin: Run: touch-up godoc and minor cleanups #6468
• cli-plugins/plugin: Run: allow customizing the CLI #6475
• cli/command: add WithUserAgent option #4574

How I did it
Add support to the cli/command package to accept a custom User
Agent to pass to the underlying client.

This is used as the UpstreamClient portion of the User-Agent
when the Moby daemon makes requests.

For example, pushing and pulling images with Compose might result
in the registry seeing a User-Agent value of:

docker/24.0.7 go/go1.20.10 git-commit/311b9ff kernel/6.5.13-linuxkit os/linux arch/arm64 UpstreamClient(docker-cli-plugin-compose/v2.24.0)

See also:

• cli/command: newAPIClientFromEndpoint: use WithUserAgent #4363
• client: add WithUserAgent() option moby/moby#45677

- How to verify it
Build a CLI plugin against latest HEAD, update its SchemaVersion to 0.2.0, and inspect network traffic.

- Description for the changelog

Go SDK: cli/command: add `WithUserAgent` option

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 69.23077% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cli/command/cli_options.go	57.14%	2 Missing and 1 partial ⚠️
cli/command/cli.go	83.33%	0 Missing and 1 partial ⚠️

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