[Elasticsearch] Add necessary manage privileges for synthetics indices to the kibana_system user to support ILM

[dominiqueclarke]

Related to #462

Fleet uses the internal kibana_system user to install integration package assets into Elasticsearch in order to support automatic upgrades of packages without end user intervention. To support , we'll need to add extra permissions for this user.

In particular, we must add the manage index privilege on all the indices being managed by the ILM policies for Elastic Synthetics, including:

• synthetics-http-*
• synthetics-icmp-*
• synthetics-tcp-*
• synthetics-browser-*
• synthetics-browser.network-*
• synthetics-browser.screenshot-*

Background: elastic/package-spec#293
Example: elastic/elasticsearch#81811

[shahzad31]

Since during testing lifecycle, it deleted data, we can this works so marking this as POST FF Testing succesfull.