public Builder registerExecutorPlugin(final String name, final Class<? extends ExecutorPlugin> executorImpl) {
            this.builtinPluginSourceBuilder.registerExecutorPlugin(name, executorImpl);
            return this;
        }
Provides access inside Embulk through a transactional session.

<p>It is internal implementation of {@link ExecSession} although some methods, such as {@code newPlugin}, are to be
removed from {@link ExecSession} soon during Embulk v0.10 when {@link ExecSession} moves to {@code embulk-spi} from
{@code embulk-core}. Even after the removal, they stay in this {@link ExecSessionInternal} to be called through
{@link ExecInternal}.
embulk-core/src/main/java/org/embulk/spi/ExecSessionInternal.java