[deckhouse-cli] d8 cr login and --username/--password flags

[ldmonster]

What

Adds d8 cr login — a new subcommand that authenticates against a container
registry and persists the credentials to the Docker config
(~/.docker/config.json, honouring $DOCKER_CONFIG and any configured
credential helper/store), exactly like docker login.

Also adds persistent -u/--username and -p/--password flags to the
d8 cr root so one-off credentials can be passed to any command without a
prior login.

Why

• packagecmd already calls d8 delivery-kit cr login <registry> --username … --password …; this PR provides the backing command.
• CI pipelines need a way to authenticate without storing secrets in
  ~/.docker/config.json on the host (--username/--password).
• The existing long-help text pointed users at d8 login (Kubernetes OIDC),
  which is unrelated to registry auth.

Changes

File	Change
internal/cr/cmd/basic/login.go	New d8 cr login cobra command. Verifies credentials via the registry auth handshake, then writes them through docker/cli's credential store. Interactive prompts when flags are omitted; password is read without echo on a TTY.
internal/cr/internal/registry/login.go	New registry.Login domain function. Performs a registry ping + token exchange to validate credentials before persisting. Normalises Docker Hub to its canonical https://index.docker.io/v1/ key.
internal/cr/internal/registry/options.go	Adds Transport field to Options (so --insecure is honoured during login), and NewStaticKeychain for inline credentials.
internal/cr/cmd/rootflags.go	Adds persistent -u/--username and -p/--password flags; builds a static keychain when --username is set so credentials override the Docker config for the current invocation.
internal/cr/cmd/rootflagnames/names.go	Adds Username and Password constants.
internal/cr/cmd/cr.go	Registers login subcommand; fixes help text to reference d8 cr login and --username/--password instead of the unrelated d8 login.
internal/cr/README.md	Documents login, new global flags, and a login examples cheat-sheet.

Usage

# Interactive
d8 cr login registry.example.com

# Non-interactive (CI)
d8 cr login registry.example.com -u robot -p "$TOKEN"

# One-off without persisting (any command)
d8 cr pull registry.example.com/app:v1 ./app.tar -u robot -p "$TOKEN"