Add MVP of automated performance tests (#3953)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->
We have an EOY goal to to integrate with our internal automated
performance monitoring solution.

## Description
<!--- Describe your changes in detail -->
Add a new crate at `aws/sdk/benchmarks/sdk-perf/` that runs some very
basic performance tests (currently just serializing and deserializing
DDB request/responses). Also add a new ci-script that generates the
correct SDK crates, compiles the `sdk-perf` crate, and creates a shell
script that the automated performance tests invoke to kick off our
tests.

## Testing
<!--- Please describe in detail how you tested your changes -->
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->
I deployed this code to my personal Catapult instance (along with the
changes to our internal CDK that will be CRed separately) and
successfully ran an `e2e-test`.

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
No customer visible changes, this is strictly for our CI.

----

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