Add a Runtime API client

[bmoffatt]

Description of changes:

This change adds support for using the Lambda Runtime API. The effect of this, is that Lamdba Functions written in go, can be used with both the go1.x, and provided, runtimes.

Why?

• This is a future-looking change. The Runtime API is where future innovations are going to happen.
• This makes it easier to implement custom runtimes using go. Got an idea for a custom runtime? You can re-use the AWS Lambda Go programming model to get started

How to try it:

1. Compile your function as normal.
2. When creating the .zip, name the binary bootstrap, or place a symlink to your binary named bootstrap within the .zip
3. When creating a function, choose provided as the runtime

GOOS=linux go build -o bootstrap main.go && zip lambda.zip bootstrap
aws lambda create-function \
   ... \ # iam role, name, memory size
   --zip-file lambda.zip \
   --runtime provided \
   --handler bootstrap # can be any value, the provided runtime ignores this, and always looks for an exe named `bootstrap`

Functions build this way are still compatible with the go1.x runtime. The RPC or the Runtime API mechanism will be detected on startup using environment variables Lambda defines by default.

I've also added a build flag, that allows disabling RPC support, which may result in slightly smaller binary sizes.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-commenter]

Codecov Report

Merging #298 into master will increase coverage by 3.05%.
The diff coverage is 75.65%.

@@            Coverage Diff             @@
##           master     #298      +/-   ##
==========================================
+ Coverage   74.60%   77.66%   +3.05%     
==========================================
  Files          13       17       +4     
  Lines         638      761     +123     
==========================================
+ Hits          476      591     +115     
+ Misses        123      116       -7     
- Partials       39       54      +15     

Impacted Files	Coverage Δ
lambda/rpc.go	63.63% <63.63%> (ø)
lambda/invoke_loop.go	68.51% <68.51%> (ø)
lambda/entry.go	72.22% <72.72%> (+72.22%)	⬆️
lambda/runtime_api_client.go	77.77% <77.77%> (ø)
lambda/errors.go	95.23% <95.23%> (ø)
lambda/function.go	88.37% <100.00%> (+20.63%)	⬆️
... and 2 more

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[carlzogh]

Could re-use apiVersion instead of 2018-06-01