feat: introduce metal agent mode

[utkuozdemir]

Introduce a new operating mode called the metal agent mode.

The mode is activated by the presence of a /usr/etc/is-metal-agent file under the root FS.

In this mode, Talos will:

• Only run the Initialize sequence, won't follow it up with the install/boot sequences
• Mark META/STATE partitions as missing, so they will always be detected as in "not installed" state.
• Block applying configuration via API while in maintenance mode.

This mode can be used e.g., to collect hardware information from bare-metal servers.