Colon is not allowed in column qualifiers when specifying `columns` argument

#### Problem

Although a colon is valid character in column qualifiers, `cbt` command doesn't allow it.

#### Environment details

- Programming language:
- OS: MacOS
- Language runtime version:
- Package version: 1.22.0 72e0126 2024-08-16 17:09:27

#### Steps to reproduce

1. run the following `cbt` command
```bash
# where a is the column family and b:c is the column qualifier
$ cbt -project foo -instance bar lookup baz "qux" columns="a:b:c"
2026/01/02 20:49:02 -creds flag unset, will use gcloud credential
2026/01/02 20:49:03 bad format for column "a:b:c"
```