fix(ec2): instance resourceSignalTimeout overwrites initOptions.timeout (#31446)

### Issue # (if applicable)

Closes #30052 

### Reason for this change
When specifiying both `resourceSignalTimeout` and `initOptions.timeout` in the options for creating an EC2 Instance, only the value from `resourceSignalTimeout` is used.

### Description of changes
- If both `initOptions.timeout` and `resourceSignalTimeout` are set, timeout consisting of the sum of the values and a warning suggesting that only one field should be specified
- Else, timeout is set to field specified, else defaults to 5 min

### Description of how you validated changes
- Update integration test for `instance-init.js` with both fields and verify that warning is logged
- Add unit tests:
  - `initOptions.timeout` and `resourceSignalTimeout` are both not set, timeout is set to default of 5 min
  - `initOptions.timeout` set and `resourceSignalTimeout` not set, timeout is set to initOptions.timeout
  - `initOptions.timeout` not set and `resourceSignalTimeout` is set, timeout is set to `resourceSignalTimeout`
  - `initOptions.timeout` and `resourceSignalTimeout` are both set, timeout is set to sum of timeouts and warning is logged

### Checklist
- [ ] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: michelle-wangg