Add a gradle task to list all bom dependencies (#859)

* Add a gradle task to list all bom dependencies

This can be run using `./gradlew -q :google-ads-bom:listFlattenedRuntimeDependencies`

Sample output:
```
Flattened runtime dependencies:
com.google.android:annotations
com.google.api-ads:google-ads
com.google.api-ads:google-ads-bom
com.google.api-ads:google-ads-codegen
com.google.api-ads:google-ads-stubs-lib
com.google.api-ads:google-ads-stubs-v19
com.google.api-ads:google-ads-stubs-v20
com.google.api-ads:google-ads-stubs-v21
com.google.api.grpc:proto-google-common-protos
com.google.api:api-common
com.google.api:gax
com.google.api:gax-grpc
com.google.auth:google-auth-library-credentials
com.google.auth:google-auth-library-oauth2-http
com.google.auto.service:auto-service
com.google.auto.service:auto-service-annotations
com.google.auto.value:auto-value-annotations
com.google.auto:auto-common
com.google.cloud:google-cloud-shared-dependencies
com.google.cloud:libraries-bom
com.google.code.findbugs:jsr305
com.google.code.gson:gson
com.google.errorprone:error_prone_annotations
com.google.guava:failureaccess
com.google.guava:guava
com.google.guava:listenablefuture
com.google.http-client:google-http-client
com.google.http-client:google-http-client-gson
com.google.j2objc:j2objc-annotations
com.google.protobuf:protobuf-java
com.google.protobuf:protobuf-java-util
com.google.re2j:re2j
com.squareup:javapoet
commons-codec:commons-codec
io.grpc:grpc-alts
io.grpc:grpc-api
io.grpc:grpc-auth
io.grpc:grpc-context
io.grpc:grpc-core
io.grpc:grpc-googleapis
io.grpc:grpc-grpclb
io.grpc:grpc-inprocess
io.grpc:grpc-netty-shaded
io.grpc:grpc-protobuf
io.grpc:grpc-protobuf-lite
io.grpc:grpc-services
io.grpc:grpc-stub
io.grpc:grpc-util
io.grpc:grpc-xds
io.opencensus:opencensus-api
io.opencensus:opencensus-contrib-http-util
io.perfmark:perfmark-api
javax.annotation:javax.annotation-api
org.apache.httpcomponents:httpclient
org.apache.httpcomponents:httpcore
org.checkerframework:checker-qual
org.codehaus.mojo:animal-sniffer-annotations
org.conscrypt:conscrypt-openjdk-uber
org.slf4j:slf4j-api
org.threeten:threetenbp
```

* Change task name

* Update google-ads-bom/build.gradle

Co-authored-by: Josh Radcliff <jradcliff@users.noreply.github.com>

---------

Co-authored-by: Josh Radcliff <jradcliff@users.noreply.github.com>

Author: sarahcaseybot

google-ads-bom/build.gradle

@@ -113,6 +113,9 @@ jreleaser {
     }
 }
 
+repositories {
+    mavenCentral()
+}
 
 javaPlatform {
     // Allows importing other BOMs, like the Google Cloud BOM below.
@@ -175,3 +178,22 @@ publishing {
         }
     }
 }
+
+// Lists all dependencies available in this BOM, including transitive dependencies.
+// This is useful for users to see what dependencies they can use without specifying a version.
+task listAllDependencyConstraints {
+    doLast {
+        println "Dependency constraints:"
+        def resolvable = configurations.create("resolvable")
+        resolvable.dependencies.add(dependencies.platform(project))
+        resolvable.dependencies.add(dependencies.project(path: ":google-ads"))
+
+        def allDeps = new TreeSet<String>()
+        resolvable.incoming.resolutionResult.allComponents.each { component ->
+            if (component.moduleVersion != null) {
+                allDeps.add(component.moduleVersion.id.toString())
+            }
+        }
+        allDeps.each { String dep -> println dep }
+    }
+}