Introduce encode_value/decode_value() template functions

[oxan]

What does this implement/fix?

Part of a series to clean-up helpers.h. This one is preparation for cleaning up the hexadecimal formatting/parsing helpers.

Intorudce the encode_value and decode_value template functions, as more generic versions of the pre-existing encode_uint16/encode_uint32/decode_uint16. Drop decode_uint16 and convert its user, as it's now equivalent to decode_value.

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#

Test Environment

• ESP32
• ESP32 IDF
• ESP8266

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.

[probot-esphome]

Hey there @Habbie, mind taking a look at this pull request as it has been labeled with an integration (ccs811) you are listed as a code owner for? Thanks!
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