feat(core): allow custom transport options in a browser

[sranka]

Fixes #446 . Custom transport options are herein also accepted in a browser environment.

For example: to change the credentials option from a default omit to include :

new InfluxDB({
  url,
  token,
  transportOptions: {credentials: 'include'},
})

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• yarn test completes successfully

[codecov-commenter]

Codecov Report

Merging #447 (47f2f6c) into master (3f59bcb) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #447   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           33        33           
  Lines         1234      1234           
  Branches       292       292           
=========================================
  Hits          1234      1234           

Impacted Files	Coverage Δ
packages/core/src/impl/browser/FetchTransport.ts	100.00% <ø> (ø)

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