Add transition_length in servo.write

**Describe the problem you have/What new integration you would like**

It would be nice if we could set the transition_length parameter in the servo.write action.
We could have the ability to have different rotation speed in different cases.

**Please describe your use case for this integration and alternatives you've tried:**

For example:
```
api:
services:
- service: control
variables:
level: float
transition_lenght: string
then:
- servo.write:
id: my_servo1
level: !lambda 'return (-2*(level/255))+1;'
transition_lenght: !lambda 'return transition_lenght;'

switch:
- platform: template
name: boule_ch2
restore_state: true
optimistic: true

turn_on_action:
- servo.write:
id: my_servo1
level: -1
transition_length: "00:00:10"

turn_off_action:
- servo.write:
id: my_servo1
level: 1
transition_length: "00:00:40"
```
**Additional context**

Start by locating the servo.write action entry point and the configuration path for transition_length. Use the YAML examples in the request as the acceptance cases: different fixed durations should be usable for separate actions, and the value should support the shown templated service use case.

Written by the indexing model from the issue text.