feat(parameters): add support for AppConfig Agent

[svozza]

Summary

Adds a new getConfig function exported from @aws-lambda-powertools/parameters/appconfig-agent that retrieves configurations from the AWS AppConfig Agent Lambda extension's local HTTP endpoint. Since caching, polling, and prefetching are handled by the agent, the implementation is a lean standalone function modeled on the commons metadata module rather than the BaseProvider pattern, keeping customer bundles small.

Changes

• Add getConfig function in packages/parameters/src/appconfig-agent/ that fetches from http://localhost:2772/applications/{app}/environments/{env}/configurations/{config} using native fetch with a configurable timeout (default 3000ms)
• Support json and binary transforms by reusing the existing transformValue utility; no maxAge/forceFetch options since caching is handled by the agent
• Return undefined when not running in a Lambda environment (local development/testing)
• Respect the AWS_APPCONFIG_EXTENSION_HTTP_PORT environment variable and fall back to POWERTOOLS_SERVICE_NAME when application is not provided
• Add ./appconfig-agent and ./appconfig-agent/types subpath exports with typed return values (string, JSONValue, or explicit generic)
• Add unit tests (100% coverage), type tests, and an e2e test suite that deploys the agent extension layer alongside AppConfig profiles
• Document the new function in the Parameters feature page, README, and add a doc snippet

Issue number: closes #5312

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[svozza]

E2E tests pass: https://github.com/aws-powertools/powertools-lambda-typescript/actions/runs/28928374948

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

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

See analysis details on SonarQube Cloud

[dreamorosi]

Thanks for working on this - and glad we were able to smooth out the non-Lambda experience.

[svozza]

Yeah, the non-lambda part was trickier than I expected but I think we landed on a nice solution in the end. Thank you for helping iterate on the design!