[bthome] Refactor to use FixedVector and add features

Major refactoring to address memory efficiency, advertisement cycling,
and immediate advertising support:

**Use FixedVector instead of std::vector:**
- Replace std::vector with FixedVector for measurements storage
- Initialize with exact sizes determined from configuration
- Eliminates STL reallocation overhead and reduces flash usage
- Uses runtime-sized FixedVector allocated once in setup()

**Config key changes:**
- Use `sensors` and `binary_sensors` (plurals) for consistency
- Matches ESPHome conventions for sensor arrays

**Advertisement size management and cycling:**
- Calculate max advertisement size (31 bytes total, minus overhead)
- Split measurements across multiple packets if they don't fit
- Automatically cycle through packets on each advertising interval
- Ensures all sensors get advertised even with many measurements
- Overhead: 8 bytes unencrypted, 16 bytes encrypted

**Immediate advertising support:**
- Add `advertise_immediately` option for sensors/binary_sensors
- When enabled, triggers immediate advertisement on state change
- Interrupts normal advertising cycle to send only that sensor
- Resumes normal cycle after immediate advertisement
- Perfect for motion sensors, door sensors, or critical alerts

**Implementation details:**
- Refactored encode functions to use raw pointers and calculate sizes
- Build multiple advertisement packets as needed
- Track current packet index for cycling
- Handle immediate advertising with separate packet building path
- Proper encryption handling with per-packet counters

Example configuration:
```yaml
bthome:
  sensors:
    - type: temperature
      id: room_temp
    - type: humidity
      id: room_humidity
  binary_sensors:
    - type: motion
      id: pir_sensor
      advertise_immediately: true  # Instant notification
    - type: door
      id: front_door
      advertise_immediately: true
```

tests/components/bthome/common.yaml

@@ -18,15 +18,16 @@ binary_sensor:
     name: "Test Door"
 
 bthome:
-  measurement:
+  sensors:
     - type: temperature
       id: test_temperature
     - type: humidity
       id: test_humidity
     - type: battery
       id: test_battery
-  binary_sensor:
+  binary_sensors:
     - type: motion
       id: test_motion
+      advertise_immediately: true
     - type: door
       id: test_door

tests/components/bthome/test_encryption.esp32-idf.yaml

@@ -10,9 +10,10 @@ binary_sensor:
 
 bthome:
   encryption_key: "231d39c1d7cc1ab1aee224cd096db932"
-  measurement:
+  sensors:
     - type: temperature
       id: test_temperature
-  binary_sensor:
+      advertise_immediately: true
+  binary_sensors:
     - type: motion
       id: test_motion