chore: Update ECS Exports

[gabedos]

What does this PR do?

Wraps all of the ECS exports into 1 index within the src/ecs submodules. Then exports those into the package level src/index.ts.

This avoid needing to get reviews from src/index.ts file owners.

Testing Guidelines

yarn build and ensuring all samples still worked.

Note that yarn build fails if any of the needed sub-interfaces are not directly exported. We know we are exporting everything needed for the DatadogECSFargateTaskDefinition construct because the build suceeds.

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog

[lym953]

index.ts LGTM