Add Auto Scaling Plans service client

[ewbankkit]

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Prerequisite for #6562.

Release note for CHANGELOG:

NONE

Output from acceptance testing:

$ make testacc TEST=./aws/ TESTARGS='-run=TestAccAWSProvider_'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws/ -v -parallel 20 -run=TestAccAWSProvider_ -timeout 120m
=== RUN   TestAccAWSProvider_Endpoints
=== PAUSE TestAccAWSProvider_Endpoints
=== RUN   TestAccAWSProvider_Endpoints_Deprecated
=== PAUSE TestAccAWSProvider_Endpoints_Deprecated
=== CONT  TestAccAWSProvider_Endpoints
=== CONT  TestAccAWSProvider_Endpoints_Deprecated
--- PASS: TestAccAWSProvider_Endpoints_Deprecated (3.10s)
--- PASS: TestAccAWSProvider_Endpoints (3.15s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	3.223s

[bflad]

Thanks so much, @ewbankkit 🚀 LGTM

[ghost]

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