Add clean_session as configurable option to the MQTT component

[victorclaessen]

What does this implement/fix?

Add support to configure the clean_session flag for the MQTT connection.
The underlying MQTT library already supported it, it was just not configurable for the user.

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

Related issue or feature (if applicable):

• fixes MQTT persistence? (or am I missing a point?) issues#1719

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

• Add clean_session as configurable option to the MQTT component esphome-docs#4284

Test Environment

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

Example entry for config.yaml:

mqtt:
  clean_session: 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 53.98%. Comparing base (4d8b5ed) to head (b22d2aa).
Report is 1367 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #7501      +/-   ##
==========================================
+ Coverage   53.70%   53.98%   +0.27%     
==========================================
  Files          50       50              
  Lines        9408     9688     +280     
  Branches     1654     1711      +57     
==========================================
+ Hits         5053     5230     +177     
- Misses       4056     4127      +71     
- Partials      299      331      +32     

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