feat: load protos from JSON, grpc-fallback support, IAM protos

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Executing /tmpfs/src/git/autosynth/working_repo/synth.py.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:0e6f3a668cd68afc768ecbe08817cf6e56a0e64fcbdb1c58c3b97492d12418a1
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis.
synthtool > Running generator for google/bigtable/artman_bigtable.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/js/bigtable-v2.
synthtool > Running generator for google/bigtable/admin/artman_bigtableadmin.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/js/bigtable-admin-v2.
synthtool > Replaced '../../package.json' in src/v2/bigtable_client.js.
synthtool > Replaced '../../package.json' in src/v2/bigtable_instance_admin_client.js.
synthtool > Replaced '../../package.json' in src/v2/bigtable_table_admin_client.js.
.eslintignore
.eslintrc.yml
.github/ISSUE_TEMPLATE/bug_report.md
.github/ISSUE_TEMPLATE/feature_request.md
.github/ISSUE_TEMPLATE/support_request.md
.jsdoc.js
.kokoro/common.cfg
.kokoro/continuous/node10/common.cfg
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.kokoro/presubmit/node10/system-test.cfg
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.kokoro/presubmit/node8/common.cfg
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.kokoro/presubmit/windows/test.cfg
.kokoro/publish.sh
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.kokoro/samples-test.sh
.kokoro/system-test.sh
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CODE_OF_CONDUCT.md
CONTRIBUTING.md
LICENSE
README.md
codecov.yaml
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samples/README.md

> core-js@2.6.9 postinstall /tmpfs/src/git/autosynth/working_repo/node_modules/core-js
> node scripts/postinstall || echo "ignore"

Thank you for using core-js ( https://github.com/zloirock/core-js ) for polyfilling JavaScript standard library!

The project needs your help! Please consider supporting of core-js on Open Collective or Patreon: 
> https://opencollective.com/core-js 
> https://www.patreon.com/zloirock 

Also, the author of core-js ( https://github.com/zloirock ) is looking for a good job -)


> protobufjs@6.8.8 postinstall /tmpfs/src/git/autosynth/working_repo/node_modules/protobufjs
> node scripts/postinstall


> @google-cloud/bigtable@2.0.5 prepare /tmpfs/src/git/autosynth/working_repo
> npm run compile


> @google-cloud/bigtable@2.0.5 compile /tmpfs/src/git/autosynth/working_repo
> tsc -p . && cp -r src/v2 build/src/ && cp -r proto* build/ && cp test/*.js build/test

npm notice created a lockfile as package-lock.json. You should commit this file.
added 680 packages from 1250 contributors and audited 2331 packages in 25.452s
found 0 vulnerabilities


> @google-cloud/bigtable@2.0.5 fix /tmpfs/src/git/autosynth/working_repo
> gts fix && npm run lintfix


> @google-cloud/bigtable@2.0.5 lintfix /tmpfs/src/git/autosynth/working_repo
> eslint --fix '**/*.js'

synthtool > Cleaned up 2 temporary directories.
synthtool > Wrote metadata to synth.metadata.

[codecov]

Codecov Report

Merging #536 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #536   +/-   ##
=======================================
  Coverage   97.79%   97.79%           
=======================================
  Files          10       10           
  Lines        1313     1313           
  Branches      347      347           
=======================================
  Hits         1284     1284           
  Partials       29       29

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[alexander-fenster]

@jkwlui I fixed the proto stuff but the samples test failure does not look related, what do you think?