Backport: disable no-squash with backport-squash label

[lampajr]

Changes proposed

1. Made no-squash option configurable, by default is set to true (i.e., using label backport) but if you want to force no-squash: false for any reason you can simply add label backport-squash rather than backport one.
2. Added concurrency so that if multiple jobs are triggered targeting the same PR only one will be kept alive

Check List (Check all the applicable boxes)

• My code follows the code style of this project.
• My change requires changes to the documentation.
• I have updated the documentation accordingly.
• All new and existing tests passed.

[lampajr]

@johnaohara maybe this is just temporarily needed but I think that having this option enabled could be useful for the future (don't know maybe there will be other use cases where we would like to disable no-squash)

[johnaohara]

thanks