chore(slo): Make APM indicator's index required

[kdelemme]

Summary

This PR introduces a breaking change by making the APM indicator's index required when creating an SLO. Reasoning behind it is that the apm metrics index can be different from cluster to cluster, and we are already always providing the value from the UI, so it makes sense to require it from the API standpoint.

🥼 Manual testing

You can make an API call with and without the indicator.params.index field:

curl --request POST \
  --url http://localhost:5601/kibana/api/observability/slos \
  --header 'Authorization: Basic ZWxhc3RpYzpjaGFuZ2VtZQ==' \
  --header 'Content-Type: application/json' \
  --header 'kbn-xsrf: oui' \
  --data '{
	"name": "My SLO Service Latency",
	"description": "My SLO Desc",
	"indicator": {
		"type": "sli.apm.transactionDuration",
		"params": {
			"environment": "development",
			"service": "o11y-app",
			"transactionType": "request",
			"transactionName": "GET /slow",
			"threshold": 5000,
			"filter": "",
			"index": "metrics-apm*"
		}
	},
	"timeWindow": {
		"duration": "7d",
		"isRolling": true
	},
	"budgetingMethod": "timeslices",
	"objective": {
		"target": 0.95,
		"timesliceTarget": 0.80,
		"timesliceWindow": "1m"
	}
}'

[elasticmachine]

Pinging @elastic/actionable-observability (Team: Actionable Observability)

[CoenWarmer]

LGTM!

[kibana-ci]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: c605ac8

Failed CI Steps

• FTR Configs #59

Metrics [docs]

Unknown metric groups

ESLint disabled line counts

id	before	after	diff
securitySolution	433	436	+3

Total ESLint disabled count

id	before	after	diff
securitySolution	513	516	+3

History

• 💔 Build #114093 failed 45cdd26

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

cc @kdelemme