Use core protobuf files from the source [ECR-3472] (#1177)

Protobuf specification files are now taken from a separate
repository that is incorporated into the main project as a git submodule.

git submodule init и git submodule update should be executed 
in order to fetch the content of the submodule.

See: exonum/exonum#1505

Author: skletsun

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "exonum-java-binding/common/src/main/proto"]
+	path = exonum-java-binding/common/src/main/proto
+	url = https://github.com/exonum/exonum-proto-sources.git

CONTRIBUTING.md

@@ -23,6 +23,7 @@ You need to install the following dependencies:
   To install a specific Rust version, use `rustup install 1.36.0` command.
   * The [system dependencies](https://exonum.com/doc/version/0.12/get-started/install/) of Exonum. 
   You do _not_ need to manually fetch and compile Exonum.
+
   __Important__: On Mac OS it is necessary to install RocksDB
   package and to set the environment variable `ROCKSDB_LIB_DIR`.
   To install the package via Homebrew:
@@ -36,6 +37,11 @@ You need to install the following dependencies:
   Also on Mac you need a [`coreutils`](https://formulae.brew.sh/formula/coreutils) package installed.
 
 ### Building
+__Important__: Keep in mind that project contains git submodules and extra steps are required
+to init and/or update them:
+  * `git submodule update --init` before the first build
+  * `git submodule update` when the submodule revision changes.
+
 Set required environment variables, once in a shell you use to build the project:
 ```$sh
 $ source exonum-java-binding/tests_profile

exonum-java-binding/common/pom.xml

@@ -147,6 +147,7 @@
             </goals>
             <configuration>
               <protocArtifact>com.google.protobuf:protoc:${protobuf.version}:exe:${os.detected.classifier}</protocArtifact>
+              <protoSourceRoot>${basedir}/src/main/proto/src</protoSourceRoot>
             </configuration>
           </execution>
         </executions>