[Connectors API] Implement update service type action

[jedrazb]

Changes

• Update service type action endpoint, this functionality is expected by Kibana

_connector/{connector_id}/_service_type
{
  "service_type": ....
}

• Service type change results in connector status change as well (created or needs_configuration)
• Clearing configuration is caller's responsibility via _configuration endpoint
• Add tests: unit, yml, bwc

[elasticsearchmachine]

Pinging @elastic/ent-search-eng (Team:Enterprise Search)

[timgrein]

LGTM! Potential follow-up: Should we think about a set of rules defining how you can define a service_type syntactically? In the connectors framework we seem to implicitly use snake_case. WDYT?

[jedrazb]

In the connectors framework we seem to implicitly use snake_case. WDYT?

@timgrein Yeah we follow snake_case pattern but I don't see us ever enforcing it anywhere in the connectors framework. Enforcing this on API level might break things for folks who deployed custom connectors with custom names. I prefer to not add this validation here.

[elasticsearchmachine]

Hi @jedrazb, I've created a changelog YAML for you.