fix: set element_count_limit param as optional for bundling config

[xiaozhenliu-gg5]

Fix type compile error: googleapis/gapic-generator-typescript#330

This field is required in BundlingConfig from gax, but not set in service_client_config.json from API.

[xiaozhenliu-gg5]

We have another workaround to set this number as default large if we don't specify it.

[codecov]

Codecov Report

Merging #747 into master will decrease coverage by 0.4%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #747      +/-   ##
==========================================
- Coverage   89.54%   89.14%   -0.41%     
==========================================
  Files          45       43       -2     
  Lines        7344     7211     -133     
  Branches      468      452      -16     
==========================================
- Hits         6576     6428     -148     
- Misses        765      780      +15     
  Partials        3        3

Impacted Files	Coverage Δ
src/gax.ts	96.1% <100%> (ø)	⬆️
src/fallback.ts	73.68% <0%> (-2.88%)	⬇️
src/streamingCalls/streamingApiCaller.ts	89.24% <0%> (-1.08%)	⬇️
src/grpc.ts	89.23% <0%> (-0.57%)	⬇️
samples/pagination.js
samples/quickstart.js

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