/inserts-stream endpoint does not support insertion of nested types

[vcrfxia]

Describe the bug

The /inserts-stream endpoint throws an error when trying to insert a row containing a struct column.

To Reproduce

This bug has been present since the /inserts-stream endpoint was introduced. The fix will be in 0.10.0 and 6.0.

• Create stream with struct type column
• Send request to insert row into stream via /inserts-stream, either via REST endpoint or Java client

Expected behavior

The insert should succeed.

Actual behaviour

The insert attempt fails. The exact error differs depending on whether the server contains #5469 or not. On later versions the error is

[2020-06-10 12:38:18,107] ERROR Error in processing inserts (io.confluent.ksql.api.server.AcksSubscriber:100)
io.confluent.ksql.api.server.KsqlApiException: Can't coerce a field of type class java.util.LinkedHashMap ({g1=foo}) into type STRUCT<`G1` STRING>
	at io.confluent.ksql.api.impl.KeyValueExtractor.lambda$coerceObject$0(KeyValueExtractor.java:102)
	at java.util.Optional.orElseThrow(Optional.java:290)
	at io.confluent.ksql.schema.ksql.SqlValueCoercer$Result.orElseThrow(SqlValueCoercer.java:68)
	at io.confluent.ksql.api.impl.KeyValueExtractor.coerceObject(KeyValueExtractor.java:101)
	at io.confluent.ksql.api.impl.KeyValueExtractor.extractValues(KeyValueExtractor.java:66)
	at io.confluent.ksql.api.impl.InsertsSubscriber.extractValues(InsertsSubscriber.java:193)
	at io.confluent.ksql.api.impl.InsertsSubscriber.handleValue(InsertsSubscriber.java:139)
	at io.confluent.ksql.api.impl.InsertsSubscriber.handleValue(InsertsSubscriber.java:51)
	at io.confluent.ksql.reactive.BaseSubscriber.doOnNext(BaseSubscriber.java:191)
	at io.confluent.ksql.reactive.BaseSubscriber.lambda$onNext$1(BaseSubscriber.java:67)
	at io.confluent.ksql.reactive.BaseSubscriber.runOnRightContext(BaseSubscriber.java:153)
	at io.confluent.ksql.reactive.BaseSubscriber.onNext(BaseSubscriber.java:67)
	at io.confluent.ksql.reactive.BasePublisher.doOnNext(BasePublisher.java:106)
	at io.confluent.ksql.reactive.BufferedPublisher.maybeSend(BufferedPublisher.java:149)
	at io.confluent.ksql.reactive.BasePublisher.doRequest(BasePublisher.java:182)
	at io.confluent.ksql.reactive.BasePublisher.access$200(BasePublisher.java:32)
	at io.confluent.ksql.reactive.BasePublisher$Sub.lambda$request$0(BasePublisher.java:200)
	at io.vertx.core.impl.ContextImpl.executeTask(ContextImpl.java:369)
	at io.vertx.core.impl.EventLoopContext.lambda$executeAsync$0(EventLoopContext.java:38)
	at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:164)
	at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:472)
	at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:500)
	at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:989)
	at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
	at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
	at java.lang.Thread.run(Thread.java:748)

Additional context

Prior to #5469, there was a similar issue for array and map type columns. #5469 fixed the issue for array and map but not struct.