ECS: Increase allowed values for the "ECS_RESERVED_MEMORY" parameter

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**Tell us about your request**
What do you want us to build?

- Allow the "ECS_RESERVED_MEMORY" to be set to values over 65535

**Which service(s) is this request for?**
ECS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**

- In some cases, for example where the cluster holds other memory-intensive processes, the current limit is too small to ensure that these services will run smoothly

**Are you currently working around this issue?**

- The workaround is to manage the memory used by the tasks, rather than the memory used by system processes, to ensure that enough memory is left for the system. E.g. setting the "memory" property on the task definitions for running services

**Attachments**

- Currently, the "ECS_RESERVED_MEMORY" values are constrained by [this method in the ECS official repository](https://github.com/aws/amazon-ecs-agent/blob/206fdadfca022d3f05dc4ba59599e08852d58ffd/agent/config/parse.go#L320), where the set values have the type [uint16](https://golang.org/pkg/builtin/#uint16). Changing the supported types to include uint32 would allow specifying greater memory values

Start by reading agent/config/parse.go around line 320 in the linked Amazon ECS Agent repository, focusing on how ECS_RESERVED_MEMORY is parsed and constrained. Done means the parameter accepts values above 65535 while preserving valid existing behavior; the issue does not name a test file, so locate the relevant parser tests before making changes.

Written by the indexing model from the issue text.