chore: adding linter to google-cloud-node monorepo (1/5)

[shivanee-p]

Description

This PR is the first of a series of PRs to migrate packages to standard ESLint and consolidate configurations at the repository root, addressing issue #4580 to prepare for the Librarian migration.

Trying to accomplish moving over the entire repo in 1 PR led to memory issues during analysis, so we are rolling this out in 5 batches. This PR enables and verifies standard ESLint for the first 20 packages.

Changes:

• Consolidated Configurations: Created a root .eslintrc.cjs and .eslintignore and deleted all package-level .eslintrc.json and .eslintignore files across the repository.
• Migrated 20 Packages: Enabled standard ESLint for the following packages:
  • packages/gapic-node-processing
  • packages/google-ads-admanager
  • packages/google-ads-datamanager
  • packages/google-ai-generativelanguage
  • packages/google-analytics-admin
  • packages/google-analytics-data
  • packages/google-api-apikeys
  • packages/google-api-cloudquotas
  • packages/google-api-servicecontrol
  • packages/google-api-servicemanagement
  • packages/google-api-serviceusage
  • packages/google-appengine
  • packages/google-apps-events-subscriptions
  • packages/google-apps-meet
  • packages/google-area120-tables
  • packages/google-chat
  • packages/google-cloud-accessapproval
  • packages/google-cloud-advisorynotifications
  • packages/google-cloud-aiplatform
  • packages/google-cloud-alloydb
• Baseline Violations: Added file-level /* eslint-disable @typescript-eslint/no-explicit-any */ comments to baseline historical violations of the no-explicit-any rule in these packages.
• TSConfig Updates: Updated tsconfig.json in these 20 packages to correctly extend the root gts configuration.

Verification Results:

Ran ESLint sequentially on all 20 packages and verified that they now pass cleanly (with warnings about unused variables in generated code).