[Change Proposal] Support providing a default value for `use_apm` variable in integration type packages

[kpollich]

The use_apm variable is a special-cased variable injected by Fleet for OTel input packages that support ingesting traces. We should support this variable for integration packages as well that include an otelcol input. Integration packages should be able to set a default value for this variable of true/false.

cc @nchaulet @jsoriano

[jsoriano]

use_apm behaves similarly to data_stream.dataset.

For data_stream.dataset, packages can define the variable to override the default set by Fleet. Like this one: https://github.com/elastic/integrations/blob/0123c6b4115cddb87aec21b23ddb6d83f6d5e3f7/packages/filestream/manifest.yml#L49

So in theory it should be already possible to do the same with use_apm.

Perhaps we could add validation, so when these variables are re-defined, they match what Fleet expect, and for use_apm, we only allow to use it in otelcol inputs.

In any case we should offer the same mechanisms for both special cases.