feat: support setting timeout per RPC (#379)

* feat: support setting timeout per RPC

The Spanner client allows a user to set custom timeouts while creating a
SpannerOptions instance, but these timeouts are static and are applied to
all invocations of the RPCs. This change introduces the possibility to set
custom timeouts and other call options on a per-RPC basis.

Fixes #378

* fix: change grpc deps from test to compile scope

* fix: address review comments

* fix: resolve review comment

google-cloud-spanner/pom.xml

@@ -203,6 +203,18 @@
       <groupId>com.google.api.grpc</groupId>
       <artifactId>proto-google-cloud-spanner-admin-database-v1</artifactId>
     </dependency>
+    <dependency>
+      <groupId>com.google.api.grpc</groupId>
+      <artifactId>grpc-google-cloud-spanner-admin-instance-v1</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>com.google.api.grpc</groupId>
+      <artifactId>grpc-google-cloud-spanner-v1</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>com.google.api.grpc</groupId>
+      <artifactId>grpc-google-cloud-spanner-admin-database-v1</artifactId>
+    </dependency>
     <dependency>
       <groupId>com.google.guava</groupId>
       <artifactId>guava</artifactId>
@@ -246,21 +258,6 @@
       <scope>test</scope>
     </dependency>
 
-    <dependency>
-      <groupId>com.google.api.grpc</groupId>
-      <artifactId>grpc-google-cloud-spanner-v1</artifactId>
-      <scope>test</scope>
-    </dependency>
-    <dependency>
-      <groupId>com.google.api.grpc</groupId>
-      <artifactId>grpc-google-cloud-spanner-admin-instance-v1</artifactId>
-      <scope>test</scope>
-    </dependency>
-    <dependency>
-      <groupId>com.google.api.grpc</groupId>
-      <artifactId>grpc-google-cloud-spanner-admin-database-v1</artifactId>
-      <scope>test</scope>
-    </dependency>
     <!-- Need testing utility classes for generated gRPC clients tests -->
     <dependency>
       <groupId>com.google.api</groupId>