java.lang.NoClassDefFoundError: com/google/api/gax/rpc/TransportChannelProvider

I'm getting the following error when I try to run `connect-standalone.sh`:
```
[2020-11-03 08:16:25,930] ERROR Stopping due to error (org.apache.kafka.connect.cli.ConnectStandalone:130)
java.lang.NoClassDefFoundError: com/google/api/gax/rpc/TransportChannelProvider
at java.base/java.lang.Class.getDeclaredConstructors0(Native Method)
at java.base/java.lang.Class.privateGetDeclaredConstructors(Class.java:3137)
at java.base/java.lang.Class.getConstructor0(Class.java:3342)
at java.base/java.lang.Class.newInstance(Class.java:556)
at org.apache.kafka.connect.runtime.isolation.DelegatingClassLoader.versionFor(DelegatingClassLoader.
java:395)
at org.apache.kafka.connect.runtime.isolation.DelegatingClassLoader.getPluginDesc(DelegatingClassLoad
er.java:365)
at org.apache.kafka.connect.runtime.isolation.DelegatingClassLoader.scanPluginPath(DelegatingClassLoa
der.java:337)
at org.apache.kafka.connect.runtime.isolation.DelegatingClassLoader.scanUrlsAndAddPlugins(DelegatingC
lassLoader.java:268)
at org.apache.kafka.connect.runtime.isolation.DelegatingClassLoader.registerPlugin(DelegatingClassLoa
der.java:260)
at org.apache.kafka.connect.runtime.isolation.DelegatingClassLoader.initPluginLoader(DelegatingClassL
oader.java:229)
at org.apache.kafka.connect.runtime.isolation.DelegatingClassLoader.initLoaders(DelegatingClassLoader
.java:206)
at org.apache.kafka.connect.runtime.isolation.Plugins.(Plugins.java:61)
at org.apache.kafka.connect.cli.ConnectStandalone.main(ConnectStandalone.java:79)
Caused by: java.lang.ClassNotFoundException: com.google.api.gax.rpc.TransportChannelProvider
at java.base/java.net.URLClassLoader.findClass(URLClassLoader.java:471)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:589)
at org.apache.kafka.connect.runtime.isolation.PluginClassLoader.loadClass(PluginClassLoader.java:104)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:522)
... 13 more
```
Prior to the error, some plugins seem to be loaded fine:
```
[2020-11-03 08:16:25,597] INFO Scanning for plugin classes. This might take a moment ... (org.apache.kafka.co
nnect.cli.ConnectStandalone:78)
[2020-11-03 08:16:25,637] INFO Loading plugin from: /opt/bitnami/kafka/connectors/pubsub-0.4-alpha/kafka-conn
ector/target/maven-archiver (org.apache.kafka.connect.runtime.isolation.DelegatingClassLoader:246)
[2020-11-03 08:16:25,713] INFO Registered loader: PluginClassLoader{pluginLocation=file:/opt/bitnami/kafka/co
nnectors/pubsub-0.4-alpha/kafka-connector/target/maven-archiver/} (org.apache.kafka.connect.runtime.isolation
.DelegatingClassLoader:269)
[2020-11-03 08:16:25,716] INFO Added plugin 'org.apache.kafka.connect.connector.policy.AllConnectorClientConf
igOverridePolicy' (org.apache.kafka.connect.runtime.isolation.DelegatingClassLoader:198)
[2020-11-03 08:16:25,717] INFO Added plugin 'org.apache.kafka.connect.connector.policy.PrincipalConnectorClie
ntConfigOverridePolicy' (org.apache.kafka.connect.runtime.isolation.DelegatingClassLoader:198)
[2020-11-03 08:16:25,717] INFO Added plugin 'org.apache.kafka.connect.connector.policy.NoneConnectorClientCon
figOverridePolicy' (org.apache.kafka.connect.runtime.isolation.DelegatingClassLoader:198)
[2020-11-03 08:16:25,721] INFO Loading plugin from: /opt/bitnami/kafka/connectors/pubsub-0.4-alpha/kafka-conn
ector/target/generated-sources (org.apache.kafka.connect.runtime.isolation.DelegatingClassLoader:246)
[2020-11-03 08:16:25,731] INFO Registered loader: PluginClassLoader{pluginLocation=file:/opt/bitnami/kafka/co
nnectors/pubsub-0.4-alpha/kafka-connector/target/generated-sources/} (org.apache.kafka.connect.runtime.isolat
ion.DelegatingClassLoader:269)
[2020-11-03 08:16:25,732] INFO Loading plugin from: /opt/bitnami/kafka/connectors/pubsub-0.4-alpha/kafka-conn
ector/target/generated-test-sources (org.apache.kafka.connect.runtime.isolation.DelegatingClassLoader:246)
[2020-11-03 08:16:25,737] INFO Registered loader: PluginClassLoader{pluginLocation=file:/opt/bitnami/kafka/co
nnectors/pubsub-0.4-alpha/kafka-connector/target/generated-test-sources/} (org.apache.kafka.connect.runtime.i
solation.DelegatingClassLoader:269)
[2020-11-03 08:16:25,738] INFO Loading plugin from: /opt/bitnami/kafka/connectors/pubsub-0.4-alpha/kafka-conn
ector/target/test-classes (org.apache.kafka.connect.runtime.isolation.DelegatingClassLoader:246)
[2020-11-03 08:16:25,886] INFO Registered loader: PluginClassLoader{pluginLocation=file:/opt/bitnami/kafka/co
nnectors/pubsub-0.4-alpha/kafka-connector/target/test-classes/} (org.apache.kafka.connect.runtime.isolation.D
elegatingClassLoader:269)
[2020-11-03 08:16:25,893] INFO Loading plugin from: /opt/bitnami/kafka/connectors/pubsub-0.4-alpha/kafka-conn
ector/target/classes (org.apache.kafka.connect.runtime.isolation.DelegatingClassLoader:246)
```
I'm running a Kafka cluster from [GCP Marketplace](https://console.cloud.google.com/marketplace/details/bitnami-launchpad/kafka-cluster) with the [0.4 Alpha release](https://github.com/GoogleCloudPlatform/pubsub/releases/tag/v0.4-alpha) of the Kafka connector, built with Maven 3.6.3. Here's my configuration file for the connector:
```
name=TestSinkConnector
connector.class=com.google.pubsub.kafka.sink.CloudPubSubSinkConnector
tasks.max=1
topics=test
cps.topic=projects//topics/
cps.project=
```