zshell: add Zephyr-compatible obscured input and root command selection

[swoisz]

Summary

Add the missing Zephyr-compatible shell primitives needed for applications to
implement login/logout flows while keeping authentication policy outside
zshell.

This is a compatibility feature, not a request for zshell to own passwords,
hashing, credential storage, or authorization policy.

Upstream Zephyr behavior

Zephyr documents a login pattern composed from general shell features:

• CONFIG_SHELL_START_OBSCURED and shell_obscure_set() obscure entered
  characters.
• CONFIG_SHELL_CMD_ROOT and shell_set_root_cmd() restrict command dispatch
  to a selected root command.
• CONFIG_SHELL_PROMPT_CHANGE and shell_prompt_change() switch between login
  and normal prompts.
• The application supplies the login/logout commands and credential
  verification.

References:

• https://docs.zephyrproject.org/latest/services/shell/index.html#obscured-input-feature
• https://github.com/zephyrproject-rtos/zephyr/blob/main/samples/subsys/shell/shell_module/src/login_cmd.c
• https://github.com/zephyrproject-rtos/zephyr/blob/main/samples/subsys/shell/shell_module/prj_login.conf
• https://docs.zephyrproject.org/latest/doxygen/html/group__shell__api.html

Proposed Boreas surface

Add Zephyr-compatible APIs:

int shell_obscure_set(const struct shell *sh, bool obscure);
int shell_prompt_change(const struct shell *sh, const char *prompt);
int shell_set_root_cmd(const char *cmd);

Add analogous Boreas Kconfig options using the existing ZSHELL namespace:

CONFIG_ZSHELL_START_OBSCURED
CONFIG_ZSHELL_PROMPT_CHANGE
CONFIG_ZSHELL_PROMPT_BUFF_SIZE
CONFIG_ZSHELL_CMD_ROOT

CONFIG_ZSHELL_CMDS_SELECT and the user-facing select command may be included
for broader parity, but they are not required for the core login/logout
composition if runtime root selection is otherwise supported.

Behavioral requirements

Obscured input

• When obscured, printable input is represented by * rather than the original
  character.
• Backspace and cursor behavior remain coherent.
• shell_obscure_set() returns the previous state, matching Zephyr's API
  contract.
• Startup state honors CONFIG_ZSHELL_START_OBSCURED.

Root command selection

• A configured or runtime-selected root command becomes the effective root for
  parsing.
• Input is passed as arguments beneath the selected command, matching Zephyr's
  login sample.
• Other root commands cannot be executed while a root is selected.
• Help and tab completion must not expose commands outside the selected root.
• Passing NULL to shell_set_root_cmd() restores the full root command tree.
• Invalid command names return -EINVAL.
• The selection applies consistently to all shell instances, matching the
  Zephyr API contract.

Prompt changes

• shell_prompt_change() updates the prompt at runtime.
• Prompt storage is bounded by CONFIG_ZSHELL_PROMPT_BUFF_SIZE.
• Invalid or oversized prompts return -EINVAL.
• Existing static/default prompt behavior remains unchanged when runtime prompt
  changes are disabled.

History

Provide a safe, documented way for an application login handler to purge the
credential-bearing history entry after authentication. The upstream sample
explicitly purges history before enabling the full command tree.

Tests

Add host tests covering:

• obscured character insertion and deletion;
• initial obscured state from Kconfig;
• valid and invalid runtime root selection;
• dispatch beneath a selected root;
• rejection of unrelated root commands;
• root-aware help/completion behavior;
• clearing the selected root;
• runtime prompt changes and bounds;
• history purge;
• unchanged behavior when the new options are disabled.

A small example modeled on Zephyr's login/logout sample would make the intended
composition and compatibility goal clear without embedding any authentication
policy in zshell.