"scheduleExpression" property for Timer triggers should fallback to "schedule"

If you create a `function.json` or a `FunctionMetadata` for timer trigger with a property `scheduleExpression`, it throws errors as host expects it to be `schedule` instead.

This is somewhat an issue, because the property inside the [TimerTriggerAttribute](https://github.com/Azure/azure-webjobs-sdk-extensions/blob/dev/src/WebJobs.Extensions/Extensions/Timers/TimerTriggerAttribute.cs#L44) is called `ScheduleExpression`. This is especially tricky for .NET 5 worker where we use Source Generation to convert these attribute properties to `FunctionMetadata`.

This also makes sense just for parity. We should add a fallback so that either `schedule` or `scheduleExpression` is deemed correct in this context.

Start with the TimerTriggerAttribute.cs reference in the issue and trace how timer-trigger FunctionMetadata reads schedule properties in the host. Verify that metadata using either schedule or scheduleExpression is accepted without the current error; add or update coverage where the existing timer-trigger validation is located.

Written by the indexing model from the issue text.