Move beacon transmit rate configuration from radio level to BSS level

Move beacon transmit rate configuration from radio level to BSS level

This enables control of the beacon transmission rate on a per-BSS basis.
Refactor beacon rate handling by moving the beacon_rate and rate_type
fields from struct hostapd_config to the per-BSS struct
hostapd_bss_config structure. This change ensures that beacon rate
settings are applied at BSS level, allowing multiple BSS instances to
have independent configurations. This updates the configuration parsing
logic to set these values in BSS context and adjusts beacon parameter
building to reference the BSS-level fields.

Signed-off-by: Haribabu Krishnasamy <hkr@qti.qualcomm.com>