Self instumentation config mismatch for "api_key" and "secret_token" in libbeat instrumentation

[1pkg]

This is a followup issue discovered after #13597. When APM Server is self-instrumented in the managed mode some of the APMConfig ("api_key" and "secret_token") sent by Elastic Agent are getting ignored because of the additional marshaling that erase original config keys here.

This will break self-instrumentation setup when "api_key" or "secret_token" authentication is configured. Therefore we should fix the mismatched config propagation to libbeat instrumentation and make sure that the authentication works as expected.