[APM] Broad version support for custom package policy editors in APM Fleet integration

[ogupte]

The custom APM integration settings (create_apm_policy_form.tsx / edit_apm_policy_form.tsx) at /app/fleet/integrations/apm/add-integration, should be enhanced to support a broad range of apmpackage versions.

It's not uncommon for outdated or newer packages to be in use and could very well be out of sync with the stack version. It's also possible that new package versions that become available on the EPR are automatically updated on an agent policy, and we need to be tolerant of these cases.

The customized settings should include an "Other settings" catch-all, generic section for all input vars that are not already included in the above custom settings. There should also be guards in place that disables or hides inputs which are not supported in the APM package version that is currently being edited. The user should already be aware that an updated version exists since we set the useLatestPackageVersion setting on the edit UI where a section should be disabled. The create UI could simply hide these sections, since it's an advanced use case when the user is intentionally installing an outdated package version.

The current UI for fleet migration via the APM schema settings is hardcoded and should always use the latest available package (https://github.com/elastic/kibana/blob/main/x-pack/plugins/apm/common/fleet.ts#L11)

[elasticmachine]

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

[dannycroft]

Closing as the issue is marked as done.