regex support for branchcleaner prow plugin

[ashwindasr]

What would you like to be added:
Support for regex for branches in allow map

Why is this needed:
The branchcleaner plugin automatically deletes source branches for merged PRs between two branches on the same repository. Branches in the configured allow map would be exempt from branch garbage collection even if the branches are already merged into the target branch. But right now we have to explicitly list the exclusion branches. Having regex support will enable to exclude future branches as well, if it follows the regex pattern allowing for more flexibility.

[ashwindasr]

/sig testing

[ashwindasr]

/cc @Colstuwjx

[liangyuanpeng]

/area prow

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