fix: change options to config based on BasePlugin

[OlivierAlbertini]

Which problem is this PR solving?

• closes plugins don't get config #313

Short description of the changes

• Keep initialization in ctor in order to avoid undefined issue about _config property if we intend to use it outside PluginLoader. (related to Plugin config optional vs required #210)
• use _config instead of options.

[codecov-io]

Codecov Report

Merging #314 into master will decrease coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #314      +/-   ##
==========================================
- Coverage      98%   97.93%   -0.07%     
==========================================
  Files         101       98       -3     
  Lines        4912     4455     -457     
  Branches      412      384      -28     
==========================================
- Hits         4814     4363     -451     
+ Misses         98       92       -6

Impacted Files	Coverage Δ
packages/opentelemetry-plugin-http/src/http.ts	96.34% <100%> (ø)	⬆️
...ckages/opentelemetry-plugin-grpc/test/grpc.test.ts	95.97% <100%> (ø)	⬆️
...-plugin-http/test/integrations/http-enable.test.ts	91.91% <100%> (ø)	⬆️
...y-plugin-http/test/functionals/http-enable.test.ts	98.17% <100%> (ø)	⬆️
packages/opentelemetry-plugin-grpc/src/grpc.ts	96.23% <100%> (ø)	⬆️
...ckages/opentelemetry-core/src/common/NoopLogger.ts	33.33% <0%> (-9.53%)	⬇️
.../opentelemetry-core/test/metrics/NoopMeter.test.ts	88.88% <0%> (-5.99%)	⬇️
...core/src/context/propagation/BinaryTraceContext.ts	97.67% <0%> (-0.69%)	⬇️
...entelemetry-core/test/common/ConsoleLogger.test.ts	100% <0%> (ø)	⬆️
...core/src/context/propagation/NoopHttpTextFormat.ts	100% <0%> (ø)	⬆️
... and 25 more

[draffensperger]

Why protected and not private?

[OlivierAlbertini]

Why protected and not private?

It's protected on BasePlugin.