UdfSchemaProvider doesn't work with Structs

[agavra]

Describe the bug

@UdfSchemaProvider should be usable with UDFs that return Struct, but it fails to register such UDFs.

To Reproduce

The following UDF fails to register:

  @UdfDescription(
      name = "KsqlStructUdf",
      description = "A test-only UDF for testing struct return types")
  public static class KsqlStructUdf {

    private static final SqlStruct RETURN =
        SqlStruct.builder().field("VAL", SqlTypes.STRING).build();

    @UdfSchemaProvider
    public SqlType provide(final List<SqlType> params) {
      return RETURN;
    }

    @Udf(schemaProvider = "provide")
    public Struct getDecimalStruct() {
      return null;
    }
  }

Expected behavior
Should register

Actual behaviour

[2020-05-05 15:11:23,955] WARN Failed to add UDF to the MetaStore. name=KsqlStructUdf method=public org.apache.kafka.connect.data.Struct io.confluent.ksql.function.UdfLoaderTest$KsqlStructUdf.getDecimalStruct() (io.confluent.ksql.function.UdfLoader:113)
io.confluent.ksql.util.KsqlException: Could not load UDF method with signature: public org.apache.kafka.connect.data.Struct io.confluent.ksql.function.UdfLoaderTest$KsqlStructUdf.getDecimalStruct()
	at io.confluent.ksql.function.FunctionLoaderUtils.getReturnType(FunctionLoaderUtils.java:176)
	at io.confluent.ksql.function.FunctionLoaderUtils.getReturnType(FunctionLoaderUtils.java:161)
	at io.confluent.ksql.function.UdfLoader.createFunction(UdfLoader.java:148)
	at io.confluent.ksql.function.UdfLoader.loadUdfFromClass(UdfLoader.java:106)
	at io.confluent.ksql.function.UserFunctionLoader.loadFunctions(UserFunctionLoader.java:122)
	at io.confluent.ksql.function.UserFunctionLoader.load(UserFunctionLoader.java:84)
	at io.confluent.ksql.function.UdfLoaderTest.initializeFunctionRegistry(UdfLoaderTest.java:1293)
	at io.confluent.ksql.function.UdfLoaderTest.<clinit>(UdfLoaderTest.java:98)
	at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
	at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
	at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
	at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
	at org.junit.runners.BlockJUnit4ClassRunner.createTest(BlockJUnit4ClassRunner.java:250)
	at org.junit.runners.BlockJUnit4ClassRunner.createTest(BlockJUnit4ClassRunner.java:260)
	at org.junit.runners.BlockJUnit4ClassRunner$2.runReflectiveCall(BlockJUnit4ClassRunner.java:309)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.BlockJUnit4ClassRunner.methodBlock(BlockJUnit4ClassRunner.java:306)
	at org.junit.runners.BlockJUnit4ClassRunner$1.evaluate(BlockJUnit4ClassRunner.java:100)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:366)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:103)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:63)
	at org.junit.runners.ParentRunner$4.run(ParentRunner.java:331)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:79)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:329)
	at org.junit.runners.ParentRunner.access$100(ParentRunner.java:66)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:293)
	at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:413)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
	at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
	at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)
	at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:230)
	at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:58)
Caused by: io.confluent.ksql.util.KsqlException: Type inference is not supported for: class org.apache.kafka.connect.data.Struct
	at io.confluent.ksql.execution.function.UdfUtil.handleParameterizedType(UdfUtil.java:138)
	at io.confluent.ksql.execution.function.UdfUtil.getSchemaFromType(UdfUtil.java:100)
	at io.confluent.ksql.function.FunctionLoaderUtils.getReturnType(FunctionLoaderUtils.java:170)
	... 32 more