chore(plugin-pg): move dev dependencies out of dependencies in package.json

[mayurkale22]

Which problem is this PR solving?

• Some of the package.json dependencies from plugin-pg module are not necessary for running this module (only for development).

/cc @markwolff

[codecov-io]

Codecov Report

Merging #645 into master will decrease coverage by 1.75%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #645      +/-   ##
==========================================
- Coverage   92.02%   90.26%   -1.76%     
==========================================
  Files         191      189       -2     
  Lines        9500     9638     +138     
  Branches      854      880      +26     
==========================================
- Hits         8742     8700      -42     
- Misses        758      938     +180

Impacted Files	Coverage Δ
packages/opentelemetry-core/test/utils/url.test.ts	50% <0%> (-50%)	⬇️
...pentelemetry-core/test/internal/validators.test.ts	50% <0%> (-50%)	⬇️
...elemetry-core/test/trace/spancontext-utils.test.ts	55.55% <0%> (-44.45%)	⬇️
...lemetry-core/test/trace/ProbabilitySampler.test.ts	56.52% <0%> (-43.48%)	⬇️
...s/opentelemetry-core/test/trace/NoopTracer.test.ts	60% <0%> (-40%)	⬇️
...s/opentelemetry-core/test/context/B3Format.test.ts	63.39% <0%> (-36.61%)	⬇️
...ges/opentelemetry-core/test/trace/NoopSpan.test.ts	63.63% <0%> (-36.37%)	⬇️
...s/opentelemetry-core/test/trace/tracestate.test.ts	65.06% <0%> (-34.94%)	⬇️
...ntelemetry-core/test/trace/NoRecordingSpan.test.ts	71.42% <0%> (-28.58%)	⬇️
...ackages/opentelemetry-core/src/platform/node/id.ts	71.42% <0%> (-28.58%)	⬇️
... and 39 more