Support template for outputs.cloudwatch namespace

### Use Case

I want to be able to set the namespace for cloudwatch metrics based on a tag. I have multiple metrics that are tagged with the cloudwatch namespace they need to be sent to. At the moment I have an output per namespace, this results in a lot of duplication.

This will require changing how the output batches its metrics, and will result in more cloudwatch calls depending on the cardinality of that tag.

### Expected behavior

```toml
[[outputs.cloudwatch]]
namespace = '{.Tag "namespace"}'
```

results in metrics being sent to the namespace in the tag

### Actual behavior

cloudwatch namespace is static

### Additional info

_No response_

Start by locating the outputs.cloudwatch implementation and its metric batching logic. Trace how the namespace setting is resolved, then verify that a namespace tag can determine the destination and that batching and CloudWatch calls handle multiple namespaces as described.

Written by the indexing model from the issue text.