Batch: Service implementation

## Overview
Implement AWS Batch service emulator.

## Target APIs
- `CreateComputeEnvironment`
- `DeleteComputeEnvironment`
- `DescribeComputeEnvironments`
- `CreateJobQueue`
- `DeleteJobQueue`
- `DescribeJobQueues`
- `RegisterJobDefinition`
- `SubmitJob`
- `DescribeJobs`
- `TerminateJob`

## References
- https://docs.aws.amazon.com/batch/latest/APIReference/