[outputs.cloudwatch] Make metric field separator configurable

### Use Case

The currently reported metrics look like:
```
MetricName_field1
MetricName_field2
```
Would like to be able to get metrics reported in the following format with a configuration like `metric_separator = "."`:
```
MetricName.field1
MetricName.field2
```

### Expected behavior

Would like to be able to configure separator like it is done for Wavefront output plugin: https://github.com/influxdata/telegraf/blob/a72550a02094baf939b1881a225af9763ff2cc3d/plugins/outputs/wavefront/README.md?plain=1#L41

### Actual behavior

Currently, separator is hardcoded as `_` https://github.com/influxdata/telegraf/blob/c0b3dd489e370e92aa399c42f0fca3e24b71e49d/plugins/outputs/cloudwatch/cloudwatch.go#L78

### Additional info

_No response_

Start in plugins/outputs/cloudwatch/cloudwatch.go at the hardcoded underscore separator, then compare the configurable separator described in the linked Wavefront output README. Add the CloudWatch configuration needed for metric_separator and verify that configured values produce names such as MetricName.field1 instead of MetricName_field1.

Written by the indexing model from the issue text.