Add branch flag to glab repo clone command
### Problem to solve
As of now there's no flag to specify the branch to checkout by default when cloning using `glab repo clone`.
This can be quite useful when you want to clone a set of repositories locally and you to use a branch other than the main one.
### Proposal
Add a new flag for the command like -b --branch to specify the branch to checkout when cloning the repositories.
### Further details
### Links / references
issue