[bug]: support --password-stdin for piped input and preserve whitespace

[rambhardwajj]

Description

An issue with harbor login --password-stdin when password is provided via piped input.
Previously, the login flow used term.ReadPassword(---), which expects an interactive terminal (TTY). This caused failures or incorrect behavior when stdin was piped (non-interactive).

This change introduces GetSecretStdin(...) which:

Detects whether stdin is interactive or piped
Uses hidden password input for interactive terminals
Reads directly from stdin when input is piped

Steps to Reproduce

Run command:

• echo "mypassword" | harbor login example.com -u user --password-stdin
  Observe behavior

Expected Behavior

CLI should correctly read password from stdin when piped
Password should not be modified (including leading/trailing spaces)

Actual Behavior

Login may fail or behave incorrectly when stdin is piped
Password may be altered due to TrimSpace removing valid whitespace

Environment

• OS: macOS (also reproducible on other platforms)
• Tool version: harbor-cli (main branch before fix)
• Other relevant details: stdin is non-interactive when using pipe

Additional Context

Note: Running full go test ./pkg/utils may fail in sandbox environments due to attempts to write under $HOME/.harbor, which is unrelated to this change..