Add AWS SDK clients for CFN Custom Resource support

[flostadler]

This change adds the necessary AWS SDK clients (S3 and Lambda) for supporting CFN Custom Resources in aws-native.

Relates to pulumi/pulumi-cdk#109

[flostadler]

This change is part of the following stack:

• Add AWS SDK clients for CFN Custom Resource support #1805 ◀
  • Add utility functions for supporting CloudFormation CustomResources #1808
    • Add implementation for CloudFormation Custom Resource Emulator #1806
      • Expose CloudFormation Custom Resource Emulator Resource #1807

_{Change managed by git-spice.}

[github-actions]

Does the PR have any schema changes?

Looking good! No breaking changes found.
No new resources/functions.

[codecov]

Codecov Report

Attention: Patch coverage is 70.14218% with 63 lines in your changes missing coverage. Please review.

Project coverage is 46.30%. Comparing base (91e1b65) to head (6575dfa).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
provider/pkg/client/mock_s3.go	65.38%	26 Missing and 1 partial ⚠️
provider/pkg/client/mock_lambda.go	64.58%	16 Missing and 1 partial ⚠️
provider/pkg/client/lambda.go	75.55%	10 Missing and 1 partial ⚠️
provider/pkg/client/s3.go	80.00%	7 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1805      +/-   ##
==========================================
+ Coverage   45.44%   46.30%   +0.85%     
==========================================
  Files          38       42       +4     
  Lines        5886     6097     +211     
==========================================
+ Hits         2675     2823     +148     
- Misses       2988     3047      +59     
- Partials      223      227       +4     

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