Allow underlying beats module configuration overrides in Elastic Agent from Fleet integration configuration page

[geekpete]

Describe the feature:

Currently unless a particular configuration input is surfaced in the Add Integration or specific Integration's settings page, then the configuration cannot be manually set in the resulting configuration files for particular Beats/Modules within the running Agent on the host.
Even if a configuration CAN be set, it's only set in one specific way when generated, even if there's multiple ways to set the same configuration in a resulting YAML configuration.

There's no way to provide overrides to the configuration that's generated from the available inputs or to set any other configuration that isn't handled by the available inputs in the Fleet/Integrations UI.

Describe a specific use case for the feature:

The feature is to provide a way to manually manipulate the resulting config files as an override mechanism, set from within a section in the Integration configuration page.

The purpose can be for a variety of reasons, such as workarounds or troubleshooting/testing that involve custom edits to the resulting yaml file that are not possible today with the restriction on only using values from the available configuration inputs and only in the specific resulting format that is generated even if different configuration formats might be possible for the same input data (eg single dot notation line vs nested).

Compared to Beats generally, the config files can be directly manipulated much more easily as a best practice to allow for workarounds to solve a particular requirement until a fix might be released in later stack versions, which isn't as workable when using Agent.

Such a capability might look like the way we allow overrides to the Elasticsearch output via the Advanced YAML configuration option under Fleet Management settings:
https://www.elastic.co/guide/en/fleet/8.6/fleet-agent-proxy-support.html#_set_the_proxy_for_communicating_with_elasticsearch

but will instead be specific to the config of particular Beats modules related to the particular Integration.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[cmacknz]

Done in #158699 for 8.9.0.