[uptime] Add new timestamp type for uptime sensor

[jesserockz]

What does this implement/fix?

This adds a new type field to the uptime sensor platform that allows a new type of timestamp which requires a time source to be configured and will send the timestamp of the boot once the time is syncronised for the first time. This gets the current timestamp of the sync and takes away the current boot millis() to get the timestamp of the boot.

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

Pull request in esphome-docs with documentation (if applicable): esphome/esphome-docs#4014

Test Environment

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

Example entry for config.yaml:

# Example config.yaml

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.