feat(gax): add API key authentication to ClientSettings

[ldetmer]

Allow gax client libraries to authenticate using API key via setApiKey method exposed from ClientSettings. Also added deduping to GRPC calls for api key headers.

Tested using LanguageServiceSettings

cc @westarle

[conventional-commit-lint-gcf[bot]]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot https://conventionalcommits.org/

[sonarcloud[bot]]

Quality Gate passed for 'gapic-generator-java-root'

Issues
57 New issues
0 Accepted issues

Measures
0 Security Hotspots
94.7% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[sonarcloud[bot]]

Quality Gate passed for 'java_showcase_integration_tests'

Issues
62 New issues
0 Accepted issues

Measures
0 Security Hotspots
84.2% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud