[rtttl] Add get_gain()

[edwardtfn]

What does this implement/fix?

This adds the capability to get the selected gain value for the RTTTL component.
This is useful when creating a gain control in the UI, so one can show the current gain in a slider and allow users to adjust it.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code quality improvements to existing code or addition of tests
• Other

Related issue or feature (if applicable): N/A

Pull request in esphome-docs with documentation (if applicable): N/A

Test Environment

• ESP32
• ESP32 IDF
• ESP8266
• RP2040
• BK72xx
• RTL87xx

Example entry for config.yaml:

output:
  # Buzzer output
  - id: buzzer_out
    platform: ledc
    pin:
      number: 21

rtttl:
  id: buzzer
  output: buzzer_out

esphome:
  on_boot:
    - priority: -100.44
      then:
        - lambda: |-
            id(buzzer).set_gain(id(buzzer_gain).state/100.0f);

number:
  - id: buzzer_gain
    name: Buzzer gain
    platform: template
    max_value: 100
    min_value: 0
    step: 1
    unit_of_measurement: "%"
    entity_category: config
    device_class: sound_pressure
    restore_value: true
    initial_value: 60
    optimistic: true
    set_action: 
      then:
        - lambda: |-
            id(buzzer).set_gain(x/100.0f);
            if (x > 0)
              id(buzzer).play("scale_up:d=32,o=5,b=100:c,c#,d#,e,f#,g#,a#,b");
            id(buzzer).dump_config();

sensor:
  - id: rtttl_gain
    name: Buzzer gain
    platform: template
    lambda: return id(buzzer).get_gain()*100;
    unit_of_measurement: "%"

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder).

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in esphome-docs.

[probot-esphome]

Hey there @glmnet, mind taking a look at this pull request as it has been labeled with an integration (rtttl) you are listed as a code owner for? Thanks!
_{^{(message by CodeOwnersMention)}}

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.02%. Comparing base (4d8b5ed) to head (e6a4250).
Report is 1483 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #7647      +/-   ##
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+ Coverage   53.70%   54.02%   +0.31%     
==========================================
  Files          50       50              
  Lines        9408     9717     +309     
  Branches     1654     1346     -308     
==========================================
+ Hits         5053     5250     +197     
- Misses       4056     4141      +85     
- Partials      299      326      +27     

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