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

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Co-authored-by: Tigran Najaryan <4194920+tigrannajaryan@users.noreply.github.com>
Co-authored-by: jack-berg <34418638+jack-berg@users.noreply.github.com>
Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>
Co-authored-by: David Ashpole <dashpole@google.com>