Feature request: cbt read cells-per-row option

**Is your feature request related to a problem? Please describe.**

I was trying to test how BigTable behaves when you send a zero cells-per-row row filter and reached for `cbt read` to construct a request but it didn't have the option.

**Describe the solution you'd like**

```
cbt read cells-per-column=n
```

**Describe alternatives you've considered**

Hand-rolling the request myself using the SDK.

Start at the `cbt read` command and trace how its row-filter options are parsed and passed into the Bigtable request. Resolve the mismatch between the title's cells-per-row wording and the example's cells-per-column wording, then add the requested option and verify that `cbt read` produces a request with the intended zero cells-per-row filter.

Written by the indexing model from the issue text.