Introduce Entities Data Model, Part 1

This is a proposal of a data model to represent entities. The purpose of the data model
is to have a common understanding of what an entity is, what data needs to be recorded,
transferred, stored and interpreted by an entity observability system.

This data model sets the foundation for adding entities to OpenTelemetry. The data model
is largely borrowed from
[the initial proposal](https://docs.google.com/document/d/1VUdBRInLEhO_0ABAoiLEssB1CQO_IcD5zDnaMEha42w/edit)
that was accepted for entities SIG formation.

This OTEP is step 1 in introducing the entities data model. Follow up OTEPs will add
further data model definitions, including the linking of Resource information
to entities.