Add e2e-cli for E2E testing

[MichaelGHSeg]

Summary

• Adds e2e-cli/ — a Go CLI that reads a JSON event sequence via --input, sends events through analytics.NewWithConfig, and outputs {"success": true/false, "sentBatches": N} to stdout
• Supports all six event types: track, identify, page, screen, alias, group
• Lives as a sub-module (github.com/segmentio/analytics-go/v3/e2e-cli) with a replace directive pointing to the parent, so it always tests the local SDK
• Adds .github/workflows/e2e-tests.yml and publish-e2e-cli.yml
• Restricts e2e-config.json to basic test suite

Test plan

• cd e2e-cli && go build -o e2e-cli-bin ./... && ./e2e-cli-bin --input '{"writeKey":"...","apiHost":"...","sequences":[...]}' exits 0 and prints JSON
• Basic E2E suite passes: cd sdk-e2e-tests && CLI_COMMAND=".../e2e-cli-bin" E2E_TEST_SUITES=basic npx vitest run
• e2e-tests.yml workflow runs clean on CI

[MichaelGHSeg]

Also added .github/workflows/ci.yml to replace the old CircleCI config — tests Go 1.21, 1.22, and 1.23 in a matrix.