[lvgl] Add start_value to bar; make values templatable and updateable

[clydebarrow]

What does this implement/fix?

Types of changes

• [x] Bugfix (non-breaking change which fixes an issue)
• [x] 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):

• fixes https://discord.com/channels/429907082951524364/1382475010186154064

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

• esphome/esphome-docs#4993

Test Environment

• [ ] ESP32
• [ ] ESP32 IDF
• [ ] ESP8266
• [ ] RP2040
• [ ] BK72xx
• [ ] RTL87xx
• [x] host

Example entry for config.yaml:

# Example config.yaml

Checklist:

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

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

• [x] Documentation added/updated in esphome-docs.

[clydebarrow]

To use the changes from this PR as an external component, add the following to your ESPHome configuration YAML file:

external_components:
  - source: github://pr#9056
    components: [lvgl]
    refresh: 1h

---

(Added by the PR bot)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 20 lines in your changes missing coverage. Please review.

Project coverage is 15.97%. Comparing base (4d8b5ed) to head (3e54ec6). Report is 2860 commits behind head on dev.

Files with missing lines	Patch %	Lines
esphome/components/lvgl/widgets/lv_bar.py	0.00%	16 Missing :warning:
esphome/components/lvgl/schemas.py	0.00%	4 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##              dev    #9056       +/-   ##
===========================================
- Coverage   53.70%   15.97%   -37.74%     
===========================================
  Files          50     1165     +1115     
  Lines        9408    48578    +39170     
  Branches     1654     5839     +4185     
===========================================
+ Hits         5053     7761     +2708     
- Misses       4056    40267    +36211     
- Partials      299      550      +251     

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