[Enhancement] - Replace log4j by a logging facade

[sabdouni]

Is your feature request related to a problem? Please describe
Since the infamous CVE-2021-44832, the security teams of different organizations are suspicious when they see log4j-core as a dependency.

For example, in my organization, we are using sl4j/logback for logging but I'm constrained to package log4j-core because I'm using this magnificent SDK :)

I think it's time to give the users of this SDK the choice of the logging framework.

I will be happy to propose a PR if this enhancement is accepted.

Describe the solution you'd like
Shift the choice of the logging framework to the users by introducing a logging facade as sl4j.

Describe alternatives you've considered
I have tried excluding log4j-core (via maven) but I got an error at runtime because the class ETConfiguration (for example) is loading directly org/apache/logging/log4j/core/LoggerContext

Caused by: java.lang.NoClassDefFoundError: org/apache/logging/log4j/core/LoggerContext
at org.apache.log4j.Logger.getLogger(Logger.java:41)
at com.exacttarget.fuelsdk.ETConfiguration.<clinit>(ETConfiguration.java:47)

Additional context
Add any other context or screenshots about the feature request here.