Option to remove v from start of custom_tag

Currently if I specify a custom_tag as `sometext-${{ someversionnumber}}` the tag that gets applied is prefixed with a `'v' character.

There doesn't seem to be the option to exclude the `'v'.

Start by locating how the GitHub Action reads custom_tag and assembles the version tag, then inspect the existing tag-generation behavior. Add a way for custom_tag users to omit the leading “v” and verify that the generated tag matches the requested format without changing the default behavior.

Written by the indexing model from the issue text.