Add tfawserr.ErrMessageContains, AWS SDK for Go v2 variant of helper in v2/awsv1shim/tfawserr

[ewbankkit]

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Adds an AWS SDK for Go v2 variant of the ErrMessageContains helper in v2/awsv1shim/tfawserr.

Relates #524.

% go test ./...
?   	github.com/hashicorp/aws-sdk-go-base/v2/internal/config	[no test files]
?   	github.com/hashicorp/aws-sdk-go-base/v2/internal/awsconfig	[no test files]
?   	github.com/hashicorp/aws-sdk-go-base/v2/internal/constants	[no test files]
?   	github.com/hashicorp/aws-sdk-go-base/v2/internal/errs	[no test files]
?   	github.com/hashicorp/aws-sdk-go-base/v2/internal/slices	[no test files]
?   	github.com/hashicorp/aws-sdk-go-base/v2/internal/test	[no test files]
?   	github.com/hashicorp/aws-sdk-go-base/v2/mockdata	[no test files]
?   	github.com/hashicorp/aws-sdk-go-base/v2/servicemocks	[no test files]
ok  	github.com/hashicorp/aws-sdk-go-base/v2	6.322s
ok  	github.com/hashicorp/aws-sdk-go-base/v2/internal/endpoints	0.901s
ok  	github.com/hashicorp/aws-sdk-go-base/v2/internal/expand	0.617s
ok  	github.com/hashicorp/aws-sdk-go-base/v2/logging	0.562s
ok  	github.com/hashicorp/aws-sdk-go-base/v2/tfawserr	0.353s
ok  	github.com/hashicorp/aws-sdk-go-base/v2/useragent	0.801s

[gdavison]

Looks good! 🚀