Enforce single config for APM agent service/env in API

[graphaelli]

Kibana version: 7.x

Describe the bug: Multiple configurations for the same service/environment in APM agent configuration can be created.

Steps to reproduce:

1. curl 'http://admin:XXX@localhost:5601/api/apm/settings/agent-configuration/new' -H 'Accept: */*' -H 'kbn-version: 7.6.0' -H 'Content-Type: application/json' --data '{"service":{},"settings":{"transaction_sample_rate":1,"capture_body":"off","transaction_max_spans":500}}'
2. curl 'http://admin:XXX@localhost:5601/api/apm/settings/agent-configuration/new' -H 'Accept: */*' -H 'kbn-version: 7.6.0' -H 'Content-Type: application/json' --data '{"service":{},"settings":{"transaction_sample_rate":1,"capture_body":"off","transaction_max_spans":500}}'

Expected behavior:

2nd call should fail.

Screenshots (if relevant):

related to #57005

[elasticmachine]

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

[sorenlouv]

@graphaelli How would you like subsequent request to be handled if the configuration already exists?

I see a few options:

1. Subsequent requests will return a 409 Conflict if the configuration already exists. By supplying the query param ?overwrite=true subsequent requests will update the existing configuration.

2. Subsequent request will overwrite the existing configuration unless ?overwrite=false is supplied.

I went for option 1. I'm wondering option 2 is better. Or it should be something third?

[sorenlouv]

Another option is to not have an overwrite flag at all, and always upsert.