Add context API

[dyladan]

Required for beta

Fixes #741

[codecov-io]

Codecov Report

Merging #792 into master will increase coverage by 1.94%.
The diff coverage is 47.05%.

@@            Coverage Diff            @@
##           master    #792      +/-   ##
=========================================
+ Coverage   92.56%   94.5%   +1.94%     
=========================================
  Files         237     211      -26     
  Lines       10435    9081    -1354     
  Branches     1013     856     -157     
=========================================
- Hits         9659    8582    -1077     
+ Misses        776     499     -277

Impacted Files	Coverage Δ
packages/opentelemetry-api/src/api/context.ts	47.05% <47.05%> (ø)
...ackages/opentelemetry-core/test/utils/wrap.test.ts	80.64% <0%> (ø)	⬆️
...emetry-exporter-stackdriver-trace/src/transform.ts	100% <0%> (ø)	⬆️
packages/opentelemetry-plugin-mysql/src/enums.ts	100% <0%> (ø)	⬆️
.../opentelemetry-plugin-dns/test/utils/assertSpan.ts	100% <0%> (ø)	⬆️
...in-postgres/opentelemetry-plugin-pg/src/version.ts	100% <0%> (ø)	⬆️
packages/opentelemetry-plugin-redis/src/enums.ts	100% <0%> (ø)	⬆️
...ages/opentelemetry-plugin-http/test/utils/utils.ts	33.33% <0%> (-26.67%)	⬇️
...postgres/opentelemetry-plugin-pg-pool/src/enums.ts	100% <0%> (ø)	⬆️
...-exporter-stackdriver-trace/test/transform.test.ts	100% <0%> (ø)	⬆️
... and 157 more

[mayurkale22]

lgtm

[obecny]

lgtm - one comment