Add Resource API

[OlivierAlbertini]

From #788 (comment)
We should implement Resource API specs.

Describe the solution you'd like

Implementing following specs:

1. https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/data-resource-semantic-conventions.md#version-attributes
2. https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/sdk-resource.md

I believe that it should have it's own package with opentelemetry/api deps.
Also in order to be sure that attributes is spec conform, we should provide function that would check
all naming convention rules (1.) for a specific Resource. This function would be used in all plugins during tests (perhaps some automation could be done but not mandatory).

Describe alternatives you've considered
Stay as is.

Please add any thoughs / Diagrams on how you see this.