Add support for half type for the http restful api

[venuswu]

Half type is widely used in the deeplearning inference, but the tf-serving doesn't support half type in the restful api, I submit a pr to solve this problem, please check.

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[venuswu]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

Please visit https://cla.developers.google.com/ to sign.

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[teamwong111]

I think you can adjust the type of input through tf.cast(), so that you don't need to add support for Half type.

[Hanyu-Jin]

Hi, I'm facing the same problem and I'd like to know when this pr will be approved.