flink-clients dependency must be provided by user.

In https://github.com/apache/beam/pull/13658 we changed flink-clients from compile to runtimeOnly. This had the unintended consequence of requiring the user to provide the flink-clients dependency themselves in order to run Flink on local mode, otherwise they will get the error below.

I'm guessing this happens because runtime dependencies are not included in Beam's generated pom.xml: https://github.com/apache/beam/blob/3d6cb85e4961585ae9c4fa3c7226916fef68998f/buildSrc/src/main/groovy/org/apache/beam/gradle/BeamModulePlugin.groovy#L1387-L1389

Caused by: java.lang.IllegalStateException: No ExecutorFactory found to execute the application.
at org.apache.flink.core.execution.DefaultExecutorServiceLoader.getExecutorFactory (DefaultExecutorServiceLoader.java:88)
at org.apache.flink.api.java.ExecutionEnvironment.executeAsync (ExecutionEnvironment.java:1043)
at org.apache.flink.api.java.ExecutionEnvironment.execute (ExecutionEnvironment.java:958)
at org.apache.beam.runners.flink.FlinkPipelineExecutionEnvironment.executePipeline (FlinkPipelineExecutionEnvironment.java:153)
at org.apache.beam.runners.flink.FlinkRunner.run (FlinkRunner.java:97)
at org.apache.beam.sdk.Pipeline.run (Pipeline.java:322)
at org.apache.beam.sdk.Pipeline.run (Pipeline.java:308)
at org.apache.beam.samples.sql.BeamSqlAvroExample.main (BeamSqlAvroExample.java:106)
at org.codehaus.mojo.exec.ExecJavaMojo$1.run (ExecJavaMojo.java:254)
at java.lang.Thread.run (Thread.java:834)