migrate AWS SDK v2 → v3

[zjawol]

Proposed changes

Migrates all AWS SDK usage from the legacy aws-sdk v2 package to the modular AWS SDK v3 (@aws-sdk/* packages). This is a prerequisite for dropping the v2 dependency and aligns the plugin with the current AWS SDK standard.

Key changes:

• AwsClientFactory — new class (src/aws-client-factory.ts) that lazily initializes and caches SDK v3 clients (AppSync, CloudFormation, CloudWatch Logs, Route53, ACM). ACM client is always pinned to us-east-1 as required by CloudFront.
• src/index.ts — all AWS API calls migrated from v2 SDK classes/types to v3 *Command pattern. AwsClientFactory is instantiated once in the constructor using fromNodeProviderChain() for credentials.
• src/get-stack-value.ts — removed (dead code, functionality covered elsewhere).
• sls appsync evaluate — new CLI command that calls the AppSync EvaluateCode and EvaluateMappingTemplate APIs to test JS resolvers and VTL templates against a context without deploying.
• sls appsync env get/set — new CLI commands to read and update AppSync runtime environment variables on a deployed API without redeploying.
• Documentation — updated README.md and doc/commands.md with quick-reference tables; added doc/testing-resolvers.md covering the evaluate workflow, fixture structure, pipeline chaining, and Jest integration examples. Node.js minimum requirement updated from v16 to v20.

Issue(s)

Closes #685

Steps to test or reproduce

1. Configure AWS credentials in your environment.
2. Deploy an AppSync API using the plugin.
3. Run the migrated commands and verify they work as before:

   sls appsync validate-schema
   sls appsync get-introspection
   sls appsync flush-cache
   sls appsync logs

4. Test the new evaluate command against a JS resolver:

   sls appsync evaluate --type Query --field getUser --function request \
   --context '{"arguments":{"id":"abc-123"}}'

5. Test the new env commands:

   sls appsync env get
   sls appsync env set --key STAGE --value production

6. get-stack-value.ts removal has no runtime impact.