chore: make valid connection properties public

Make the list of valid connection properties public, so tools that
depend on the Connection API can use this to for example generate
documentation for valid properties.

Author: olavloite

google-cloud-spanner/src/main/java/com/google/cloud/spanner/connection/ConnectionProperties.java

@@ -115,7 +115,6 @@
 import com.google.common.collect.ImmutableMap;
 import com.google.spanner.v1.DirectedReadOptions;
 import java.time.Duration;
-import java.util.Map;
 
 /**
  * Utility class that defines all known connection properties. This class will eventually replace
@@ -541,7 +540,7 @@ class ConnectionProperties {
           BooleanConverter.INSTANCE,
           Context.USER);
 
-  static final Map<String, ConnectionProperty<?>> CONNECTION_PROPERTIES =
+  static final ImmutableMap<String, ConnectionProperty<?>> CONNECTION_PROPERTIES =
       CONNECTION_PROPERTIES_BUILDER.build();
 
   /** Utility method for creating a new core {@link ConnectionProperty}. */

google-cloud-spanner/src/main/java/com/google/cloud/spanner/connection/ConnectionProperty.java

@@ -19,6 +19,7 @@
 import com.google.cloud.spanner.ErrorCode;
 import com.google.cloud.spanner.SpannerExceptionFactory;
 import com.google.common.base.Strings;
+import com.google.common.collect.ImmutableList;
 import java.util.Locale;
 import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
@@ -37,6 +38,9 @@
  * connection state is an opt-in.
  */
 public class ConnectionProperty<T> {
+  public static ImmutableList<ConnectionProperty<?>> VALID_CONNECTION_PROPERTIES =
+      ImmutableList.copyOf(ConnectionProperties.CONNECTION_PROPERTIES.values());
+
   /**
    * Context indicates when a {@link ConnectionProperty} may be set. Each higher-ordinal value
    * includes the preceding values, meaning that a {@link ConnectionProperty} with {@link