Cpp clang-format

[dsazonoff]

PR for #713 issue.
Added support of clang-format as a pre-build step (optional, useful for SDK contributors)
Added clang-format code style verification as a build step. Build will fail if SDK code is not following google code style (not applied for generated files).
Waiting for #803 to be merged in...

[agones-bot]

Build Failed 😱

Build Id: 9d5bc9f4-c065-478b-8520-cb7a184693e9

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[agones-bot]

Build Succeeded 👏

Build Id: fb4aa6e7-c008-411a-9219-397210129a9e

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

• image: gcr.io/agones-images/agones-controller:0.12.0-80c816a
• image: gcr.io/agones-images/agones-sdk:0.12.0-80c816a
• image: gcr.io/agones-images/agones-ping:0.12.0-80c816a
• Linux C++ SDK (build): agonessdk-0.12.0-80c816a-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-0.12.0-80c816a.zip

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

• https://80c816a-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/GoogleCloudPlatform/agones.git pull/855/head:pr_855 && git checkout pr_855
• helm install install/helm/agones --namespace agones-system --name agones --set agones.image.tag=0.12.0-80c816a

[agones-bot]

Build Succeeded 👏

Build Id: 517d1599-1085-4c84-8149-97e1a2bb7c4e

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

• image: gcr.io/agones-images/agones-controller:0.12.0-625fd75
• image: gcr.io/agones-images/agones-sdk:0.12.0-625fd75
• image: gcr.io/agones-images/agones-ping:0.12.0-625fd75
• Linux C++ SDK (build): agonessdk-0.12.0-625fd75-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-0.12.0-625fd75.zip

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

• https://625fd75-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/GoogleCloudPlatform/agones.git pull/855/head:pr_855 && git checkout pr_855
• helm install install/helm/agones --namespace agones-system --name agones --set agones.image.tag=0.12.0-625fd75

[dsazonoff]

Example of output (#900 ):

clang-format code style check failed for: sdks/cpp/src/agones/sdk.cc

@markmandel here is PR for #713 issue.
@roberthbailey could you help with code review, please?

[roberthbailey]

I'm not an expert on cmake (so if anyone is I'd appreciate their review) but I don't see any obvious issues with this change, and from #900 it appears to be validating that formatting has been successfully applied to the source files.

[dsazonoff]

@sylvainduchesne, could you do a code review, please?

[markmandel]

LGTM!