Update json and proto encoder to always accept None type, cleanup code / tests a little bit

[DylanRussell]

Description

This PR is a precursor to updating the SDK to use ExtendedAttributes everywhere: #5266.. I'm trying to make that PR a bit smaller / easier to review..

Update the encoder to always accept null as that is a valid value according to the spec, and encode it as an empty AnyValue.. this is in-line with the spec: open-telemetry/opentelemetry-specification#4392 - previously we would do this for Arrays but not Maps and not when None is encoded as a primitive..

Update _encode_attributes to accept _ExtendedAttribues value type instead of the old AttributeValue.

Cleanup the code and tests a little bit.

Type of change

• [ x] New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Unit tests

Does This PR Require a Contrib Repo Change?

• [x ] No.

Checklist:

• [x ] Followed the style guidelines of this project
• [ x] Changelogs have been updated
• [ x] Unit tests have been added
• [ x] Documentation has been updated

[herin049]

LGTM!