Initial Entity and Resource proposal (open-telemetry/oteps#264)

This is a proposal to address Resource and Entity data model
interactions, including a path forward to address immediate friction and
issues in the current resource specification.


The proposal includes all links and context needed to justify it, but
duplicating a snapshot here:

## Motivation

This proposal attempts to focus on the following problems within
OpenTelemetry to unblock multiple working groups:

- Allowing mutating attributes to participate in Resource ([OTEP
208](open-telemetry/oteps#208)).
- Allow Resource to handle entities whose lifetimes don't match the
SDK's lifetime ([OTEP
208](open-telemetry/oteps#208)).
- Provide support for async resource lookup
([spec#952](#952)).
- Fix current Resource merge rules in the specification, which most
implementations violate
([oteps#208](open-telemetry/oteps#208),
[spec#3382](#3382),
[spec#3710](#3710)).
- Allow semantic convention resource modeling to progress
([spec#605](#605),
[spec#559](#559),
etc).

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Co-authored-by: Tigran Najaryan <[email protected]>
Co-authored-by: jack-berg <[email protected]>
Co-authored-by: Arve Knudsen <[email protected]>
Co-authored-by: David Ashpole <[email protected]>