add support for kibana policy overrides in system tests

[leehinman]

Adds support for a new environment variable ELASTIC_PACKAGE_KIBANA_POLICY_OVERRIDES

If this variable is set and is a path to a yaml file, then the contents of the yaml file will be used for the kibana policy overrides in the system tests.

This allows you to test how integrations will behave with experimental settings.

Example

ELASTIC_PACKAGE_KIBANA_POLICY_OVERRIDES=/tmp/overrides.yml elastic-package test system

and the /tmp/overrides.yml file has the following contents:

agent:
  monitoring:
    _runtime_experimental: otel

Will result in the system test running with the agent monitoring using the otel runtime.

Closes #2975

[elasticmachine]

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