Adds a .clang-format file making Google style the default

[devjgm]

This change should probably have been part of PR #716, but I forgot.

.clang-format is a standard file that the clang-format tool(s) look for. This file will tell all clang-format invocations to by-default use the "Google" style. This means that someone running clang-format -i foo.cc within the sdks/cpp/... directory hierarchy will automatically use the correct style.

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This change is 

[agones-bot]

Build Succeeded 👏

Build Id: d35be90d-fe98-4b96-a70c-c17f2b1e9c63

The following development artifacts have been built, and will exist for the next 30 days:

• image: gcr.io/agones-images/agones-controller:0.10.0-def6823
• image: gcr.io/agones-images/agones-sdk:0.10.0-def6823
• image: gcr.io/agones-images/agones-ping:0.10.0-def6823
• Linux C++ SDK (build): agonessdk-0.10.0-def6823-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-0.10.0-def6823.zip

A preview of the website (the last 30 builds are retained):

• https://def6823-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/GoogleCloudPlatform/agones.git pull/724/head:pr_724 && git checkout pr_724
• helm install install/helm/agones --namespace agones-system --name agones --set agones.image.tag=0.10.0-def6823

[devjgm]

fyi, @markmandel

[markmandel]

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