APM integration UI rum response headers field is missing validation

[carsonip]

Kibana version: confirmed on 8.13.4 but should apply to all versions

Elasticsearch version: 8.13.4

Server OS version:

Browser version:

Browser OS version:

Original install method (e.g. download page, yum, from source, etc.): ESS

Describe the bug:

In APM integration policy UI, the rum custom http response headers input field does not have validation. If the user enters an invalid input, it will cause apm server to fail silently due to elastic/apm-server#14560 .

There are 2 improvements needed here:

• Instructions need to be clearer. The user does not know what should be put in this field when there is no example / clear instructions.
• There needs to be validation. Allowing a user to input invalid config only for them to fail is bad user experience. Add a validation property to this definition.

According to apm-server.yml, response headers should be an object:

    # Custom HTTP headers to add to RUM responses, e.g. for security policy compliance.
    #response_headers:
    #  X-My-Header: Contents of the header

Steps to reproduce:

1. start a ESS deployment
2. Go to apm integration policy UI, input "none" to the rum response headers field
3. restart apm integration on ESS
4. notice that ESS apm server endpoint becomes gray

Expected behavior:
The invalid input should be called out in the UI

Screenshots (if relevant):

Errors in browser console (if relevant):

Provide logs and/or server output (if relevant):

Any additional context:

[elasticmachine]

Pinging @elastic/obs-ux-infra_services-team (Team:obs-ux-infra_services)