Move validate task to base plugin

[hpmellema]

Background

• Moves the validate task to the base plugin and changes the Sources property to be Sources instead of JarToValidate as this task could be used to validate any set of smithy sources.
• The validate task is more generally useful that just for validating a JAR file in the smithy-jar plugin. Eventually we plan to add validation reports to this task which might be added as part of the base plugin. Because this task can be used outside of a jar context it should not live in the smithy-jar plugin.

Testing

• Executed integration tests which already test validation capability

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