Make AWS ECS Service discovery mechanism public and non-sealed (#3366)

[sbenedict-redblack]

Fixes #3366

Changes

Opens the EcsServiceDiscovery class to extension by making it public and non-sealed. This will allow consumers of the library to override it and provide custom implementations.

For example, out of the box, the library supports discovering "neighboring" nodes only from containers running in the same ECS Service. But as described in #3366, "neighboring" nodes may have to be discovered from multiple ECS Services that are part of an ECS Cluster. With this change, consumers of the library can use the decorator pattern to extend and modify the default behavior of the EcsServiceDiscovery class.

Checklist

For significant changes, please ensure that the following have been completed (delete if not relevant):

• This change follows the Akka.NET API Compatibility Guidelines.
• This change follows the Akka.NET Wire Compatibility Guidelines.
• I have reviewed my own pull request.
• Design discussion issue AWS ECS Discovery - Allow discovering "neighboring" nodes from multiple ECS Services #3366

Latest dev Benchmarks

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This PR's Benchmarks

No impact on performance as a result of this PR.

[Aaronontheweb]

LGTM - trivial change and the reasoning is sound.