feat: add duplicate json key validator

[jar-b]

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Relates hashicorp/terraform-provider-aws#33026

Adds a new ValidateNoDuplicateKeys function which walks a JSON object checking for duplicate key names. The intention is for aws-sdk-go-base to contain the core validation logic (ie. provide a JSON string, get back an error) and allow upstream consumers such as the AWS provider to wrap this function and decide whether to return the findings as a diagnostic warning or error.

Also creates a new validation sub-package and moves the one other existing validation function (ValidateRegion) into it.

% make test
go test -timeout=30s -parallel=4 ./...
?       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 5.960s
ok      github.com/hashicorp/aws-sdk-go-base/v2/diag    0.375s
ok      github.com/hashicorp/aws-sdk-go-base/v2/internal/config 0.460s
ok      github.com/hashicorp/aws-sdk-go-base/v2/internal/endpoints      0.392s
ok      github.com/hashicorp/aws-sdk-go-base/v2/internal/expand 0.646s
ok      github.com/hashicorp/aws-sdk-go-base/v2/logging 0.484s
ok      github.com/hashicorp/aws-sdk-go-base/v2/tfawserr        0.499s
ok      github.com/hashicorp/aws-sdk-go-base/v2/useragent       0.567s
ok      github.com/hashicorp/aws-sdk-go-base/v2/validation      0.514s
cd v2/awsv1shim && go test -timeout=30s -parallel=4 ./...
?       github.com/hashicorp/aws-sdk-go-base/v2/awsv1shim/v2/mockdata   [no test files]
ok      github.com/hashicorp/aws-sdk-go-base/v2/awsv1shim/v2    5.142s
ok      github.com/hashicorp/aws-sdk-go-base/v2/awsv1shim/v2/tfawserr   0.580s

[gdavison]

Looks good! 🚀

One suggestion about function naming