Added custom time horizon in GCP scaler

[Yaxhveer]

Added the custom time horizon for gcp scalar and updated the test accordingly

Checklist

• I have verified that my change is according to the deprecations & breaking changes policy
• Changelog has been updated and is aligned with our changelog requirements
• A PR is opened to update the documentation on (repo) (if applicable)
• Commits are signed with Developer Certificate of Origin (DCO - learn more)

Fixes #5453

Relates to #

[JorTurFer]

Looking good 😄
Does it make sense to support this in other stackdriver functions? For example, GetMetrics function has the time window hardcoded too:

keda/pkg/scalers/gcp/gcp_stackdriver_client.go

Lines 222 to 232 in a168022

	func (s StackDriverClient) GetMetrics(
	ctx context.Context,
	filter string,
	projectID string,
	aggregation *monitoringpb.Aggregation,
	valueIfNull *float64) (float64, error) {
	// Set the start time to 1 minute ago
	startTime := time.Now().UTC().Add(time.Minute * -2)
	// Set the end time to now
	endTime := time.Now().UTC()

[Yaxhveer]

@JorTurFer I have made the required changes in other stackdrivers too.

[JorTurFer]

/run-e2e gcp
Update: You can check the progress here

[JorTurFer]

/run-e2e gcp
Update: You can check the progress here

[JorTurFer]

LGTM!