Feature request: support dynamic request body similar to --rand-regex-url

I'm trying to create load for the enpoint which has a unique name constraint for the created entity.
So the imaginary ⚠️ syntax could be like:
```bash
oha --method PUT -H 'Content-Type: application/json' -d '{"name":"name-{{uuid}}"}' http://localhost/entity
# or
oha --method PUT -H 'Content-Type: application/json' -d '{"name":"name-[a-z][a-z][0-9]"}' http://localhost/entity
```
Following snippet obviously do not work, as well:
```bash
oha --method PUT -H 'Content-Type: application/json' -d "{\"name\":\"name-`uuidgen`\"}" http://localhost/entity
```