Run on amazon linux 2

[fabianfett]

This PR changes everything. Sorry. But growing up always comes with a pain.

• New package name: swift-lambda-runtime. The library is now called LambdaRuntime (Reevaluate name to prevent naming conflicts #11)
• The Runtime now only has one handler function. This is more in line with the go sdk. The end user still has the option to specify different handler methods by using the env variable _HANDLER
• The runtime is now intended to be used with the amazonlinux-swift project. That is why the makefile to extract Swift libraries from the Ubuntu Swift Dockerimage has been removed.
• swift-base64-kit has been added as a dependency to support future Event types.
• It is possible now to not return a result from the function (ByteBuffer should be optional in APIGateway #10)
• The synchronous interface has been removed.
• The documentation has been changed to reflect the changes.
• The documentation explains how to create a development image (zlib and openssl headers are missing when the lambda is built with swift:5.1.x #9)

[codecov]

Codecov Report

Merging #8 into master will decrease coverage by 0.57%.
The diff coverage is 76.47%.

@@            Coverage Diff             @@
##           master       #8      +/-   ##
==========================================
- Coverage   83.03%   82.46%   -0.58%     
==========================================
  Files          14       14              
  Lines         843      747      -96     
==========================================
- Hits          700      616      -84     
+ Misses        143      131      -12

Impacted Files	Coverage Δ
...ambdaRuntimeTests/Utils/MockLambdaRuntimeAPI.swift	0% <ø> (ø)
Sources/LambdaRuntime/RuntimeAPIClient.swift	89.7% <ø> (ø)
...ambdaRuntimeTests/Utils/Invocation+TestUtils.swift	100% <ø> (ø)
Sources/LambdaRuntime/Context.swift	100% <ø> (ø)
Sources/LambdaRuntime/Runtime+Codable.swift	100% <ø> (ø)
Tests/LambdaRuntimeTests/ContextTests.swift	95.23% <ø> (ø)
...mbdaRuntimeTests/Utils/Environment+TestUtils.swift	100% <ø> (ø)
Sources/LambdaRuntime/Events/APIGateway.swift	71.62% <100%> (ø)
...ests/LambdaRuntimeTests/Runtime+CodableTests.swift	90.4% <100%> (ø)
...sts/LambdaRuntimeTests/RuntimeAPIClientTests.swift	98.33% <100%> (ø)
... and 4 more

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