Add select platform for the oscillation angle (fixes #293)

[juanpebalsa]

Summary

Fixes #293.

Adds a select platform to the integration that exposes the fan's horizontal oscillation angle (30/60/90/120/140) as a dropdown entity, so it can be set from the UI again.

Why

The oscillation angle is not a native attribute of Home Assistant fan entities. Until now it could only be changed via the xiaomi_miio_fan.fan_set_oscillation_angle service. After the fan more-info card redesign in Home Assistant, the old angle buttons disappeared, leaving no UI control for the angle (see #293). This restores a first-class control.

What

• New select.py with async_setup_platform mirroring the fan platform config (host, token, optional model).
• XiaomiFanAngleSelect(SelectEntity): options are the angles supported by the model, current_option is read from the device (status().angle), and select_option calls set_angle.
• Model → device mapping mirrors fan.py (e.g. P18/P30 → P10, P15 → P11), and per-model angle lists (P5 = 30/60/90/120; P9/P10/P11/P15/P18/P30 = 30/60/90/120/140).

Config example

select:
  - platform: xiaomi_miio_fan
    name: Ventilador Dormitorio - Angulo Oscilacion
    host: 192.168.1.50
    token: !secret fan_token
    model: dmaker.fan.p18

Testing

Tested on a real dmaker.fan.p18 (Mi Smart Standing Fan 2 P18):

• Entity created with options ['30','60','90','120','140'] and correctly read the current angle.
• Selecting 90 → 120 → 30 changed the angle on the device and read back correctly each time.

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