Bluetooth: controller: enable dynamic TXP over LEGACY arch (#17731)

This commit targets solving issue #17731 over the Nordic LL LEGACY
arch of the BLE stack in Zephyr. This functionality is exposed
to the user as HCI Zephyr Command extensions

- BT_HCI_OP_VS_WRITE_TX_POWER_LEVEL
- BT_HCI_OP_VS_READ_TX_POWER_LEVEL

which enable Tx power read/write operations within BLE radio events
on a per role/connection basis.

The functionality is enabled upon the Kconfig advanced configuration
triggered by

- BT_CTLR_TX_PWR_DYNAMIC_CONTROL

depending on the enablement of Zephyr HCI vendor-specific command
extensions.

Signed-off-by: Andrei Stoica <stoica.razvan.andrei@gmail.com>