add github issue and PR templates

.github/ISSUE_TEMPLATE.md

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+### Expected behavior
+_[what you expected to happen]_
+
+### Actual behavior
+_[what actually happened]_
+
+### Steps to reproduce
+
+1. ...
+2. ...
+
+### If possible, minimal yet complete reproducer code (or URL to code)
+
+_[anything to help us reproducing the issue]_
+
+### SwiftAWSLambdaRuntime version/commit hash
+
+_[the SwiftAWSLambdaRuntime tag/commit hash]_
+
+### Swift & OS version (output of `swift --version && uname -a`)

.github/PULL_REQUEST_TEMPLATE.md

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+_[One line description of your change]_
+
+### Motivation:
+
+_[Explain here the context, and why you're making that change. What is the problem you're trying to solve.]_
+
+### Modifications:
+
+_[Describe the modifications you've done.]_
+
+### Result:
+
+_[After your change, what will change.]_