feat: add DSL markers to codegenned structure builders

[ianbotsf]

Issue #

Resolves #428

Description of changes

Calls within nested DSL functions will resolve an implicit receiver from anywhere up the hierarchy, enabling bad calls like this one:

CreateBucketRequest {
    bucket = "foo"
    grantRead = "bar"
    createBucketConfiguration {
        grantRead = "baz" // Should be prohibited
    }
}

In the above example, setting grantRead from within createBucketConfiguration should be prohibited because grantRead is a member of CreateBucketRequest.Builder, not CreateBucketConfiguration.Builder.

This change introduces a new DSL marker annotation @SdkDsl which is applied to every codegenned structure builder. Code like the above example now fails to compile.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.