aws_mwaaserverless_workflow: Add code argument

[astanfel]

Description

Follow-up to #49927. Adds the optional code argument to aws_mwaaserverless_workflow, which was deliberately deferred from the initial resource PR because it maps to a Smithy tagged union (types.Code / CodeMemberS3Location).

code is modeled as a nested block containing an s3_location block (the sole current union member), using the provider's standard union pattern — a flex.Expander/flex.Flattener implemented on codeModel. It is wired into the update path (the UpdateWorkflow API accepts Code, so it is updatable in place) and covered by a new acceptance test (TestAccMWAAServerlessWorkflow_code) and documentation.

resource "aws_mwaaserverless_workflow" "example" {
  name     = "example"
  role_arn = aws_iam_role.example.arn

  definition_s3_location {
    bucket     = aws_s3_bucket.example.id
    object_key = aws_s3_object.definition.key
  }

  code {
    s3_location {
      bucket     = aws_s3_bucket.example.id
      object_key = aws_s3_object.code.key
    }
  }
}

⚠️ Stacked on #49927

This branch is stacked on the branch for #49927 (which introduces the resource itself). Until #49927 merges, the diff below includes those commits. Once #49927 is merged, this branch will be rebased onto main and the diff will reduce to just the code support. Opening as a draft for early visibility; it should be reviewed after #49927.

Relations

Relates to #45254
Depends on #49927

Output from Acceptance Testing

Acceptance tests require a live account with MWAA Serverless access and are not run in this environment. The package builds, vets, and links cleanly.

$ make testacc TESTS=TestAccMWAAServerlessWorkflow_code PKG=mwaaserverless
# (pending run against an account with MWAA Serverless enabled)

[github-actions]

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