Add opentelemetry/types package

[mayurkale22]

A separate package with only TS interfaces and enums. This is based on #3 and discussion in SIGs meeting. This is also useful if some vendors want to use only types and build their API/Implementations.

It would be nice to split this ask in multiple PRs. Maybe, first PR is to add initial package structure with README, package.json, LICENSE etc (same as https://github.com/open-telemetry/opentelemetry-node/pull/2/files) and incremental PRs to add interfaces and enums.