refactor(typescript): various fixes/refactors to types

[callmehiphop]

Fixes #507

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[codecov]

Codecov Report

Merging #515 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #515      +/-   ##
==========================================
+ Coverage   94.91%   94.95%   +0.03%     
==========================================
  Files          15       15              
  Lines         984      991       +7     
  Branches       83       87       +4     
==========================================
+ Hits          934      941       +7     
  Misses         42       42              
  Partials        8        8

Impacted Files	Coverage Δ
src/publisher.ts	100% <ø> (ø)	⬆️
src/iam.ts	100% <100%> (ø)	⬆️
src/index.ts	100% <100%> (ø)	⬆️
src/snapshot.ts	100% <100%> (ø)	⬆️
src/topic.ts	91.66% <100%> (-0.2%)	⬇️
src/subscription.ts	100% <100%> (ø)	⬆️

---

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[JustinBeckwith]

LGTM when tests pass. Thank you so much for doing this!

[codecov]

Codecov Report

Merging #515 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #515      +/-   ##
==========================================
+ Coverage   94.91%   94.95%   +0.03%     
==========================================
  Files          15       15              
  Lines         984      991       +7     
  Branches       83       87       +4     
==========================================
+ Hits          934      941       +7     
  Misses         42       42              
  Partials        8        8

Impacted Files	Coverage Δ
src/publisher.ts	100% <ø> (ø)	⬆️
src/iam.ts	100% <100%> (ø)	⬆️
src/index.ts	100% <100%> (ø)	⬆️
src/snapshot.ts	100% <100%> (ø)	⬆️
src/topic.ts	91.66% <100%> (-0.2%)	⬇️
src/subscription.ts	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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