[AWS::ApplicationSignals::ServiceLevelObjective] - [BUG] - MetricDataQuery doesn't support `Period` attribute

### Name of the resource

Other

### Resource Name

AWS::ApplicationSignals::ServiceLevelObjective

### Issue Description

The `Period` attribute on MetricDataQuery should be supported, as-per the [Application Signals API documentation](https://docs.aws.amazon.com/applicationsignals/latest/APIReference/API_MetricDataQuery.html#applicationsignals-Type-MetricDataQuery-MetricStat). Without this, it doesn't seem possible to create an expression based MetricDataQuery:

error: `At least one metricStat or expression in SliConfig should define a period.`

### Expected Behavior

a `Period` attribute should be accepted in a `MetricDataQuery` object.

### Observed Behavior

a `Period` attribute is not accepted in a `MetricDataQuery` object.

### Test Cases

A MetricDataQuery object with just an `Id`, `Expression`, `ReturnData` and `Period` should be correctly accepted.

### Other Details

_No response_

Start with the linked Application Signals MetricDataQuery API documentation and the AWS::ApplicationSignals::ServiceLevelObjective resource definition; no implementation files or tests are named in the issue. Add support for Period on MetricDataQuery and verify that a query containing Id, Expression, ReturnData, and Period is accepted without the period validation error.

Written by the indexing model from the issue text.