Move MQTT ip discovery to deticated config option.

[Links2004]

What does this implement/fix?

This allows to disable entity discovery and only keep the IP discovery.

With this change its possible to discover devices with API enabled on Home Assistant (home-assistant/core#116499) when there is no mDNS possible.

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)
• Other

Pull request in esphome-docs with documentation (if applicable): esphome/esphome-docs#3809

Test Environment

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

Example entry for config.yaml:

mqtt: 
  broker: 10.225.0.16
  username: esphome
  discovery_retain: False
  discovery: False
  discover_ip: True

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.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.13%. Comparing base (4d8b5ed) to head (99881f2).
Report is 668 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #6673      +/-   ##
==========================================
+ Coverage   53.70%   54.13%   +0.42%     
==========================================
  Files          50       50              
  Lines        9408     9619     +211     
  Branches     1654     1698      +44     
==========================================
+ Hits         5053     5207     +154     
- Misses       4056     4086      +30     
- Partials      299      326      +27     

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