[APM] transactionType should be required on service-specific endpoints

[sorenlouv]

The following endpoint should require transactionType:
https://github.com/elastic/kibana/blob/master/x-pack/plugins/apm/server/routes/services.ts#L281

And the logic for determining the default transaction type should be removed. Instead the client should specify the transactionType:

kibana/x-pack/plugins/apm/server/lib/services/get_service_transaction_groups/merge_transaction_group_data.ts

Lines 36 to 44 in d4c02bb

	const transactionTypes =
	groupBucket?.transaction_types.buckets.map(
	(bucket) => bucket.key as string
	) ?? [];
	const transactionType =
	transactionTypes.find(
	(type) => type === TRANSACTION_PAGE_LOAD || type === TRANSACTION_REQUEST
	) ?? transactionTypes[0];

A general note

The logic for calculating the default transaction type resides in the client, and should not be duplicated elsewhere:
https://github.com/elastic/kibana/blob/master/x-pack/plugins/apm/public/context/apm_service_context.tsx#L32-L36

All service-specific endpoints should require transactionType and let the client specify the transactionType

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)