feat: auto-detect the value of the no-squash option

The auto-no-squash option is added to:

* backport all the commits when the pull/merge request has been merged
* backport the squashed commit otherwise

It is equivalent to dynamically adjust the value of the no-squash
option, depending on the context.

The no-squash option is kept for backward compatibility for a single
use case: backporting the merged commit instead of backporting the
commits of the pull/merge request request.

Detecting if a pull/merge request was squashed or not depends on the
underlying forge:

* Forgejo / GitHub: use the API to count the number of parents
* GitLab: if the squash_commit_sha is set, the merge request was
  squashed

If the pull/merge request is open, always backport all the commits it
contains.

Fixes: https://github.com/kiegroup/git-backporting/issues/113

Co-authored-by: Andrea Lamparelli <a.lamparelli95@gmail.com>

action.yml

@@ -85,10 +85,14 @@ inputs:
     default: "false"
   no-squash:
     description: >
-      If set to true the tool will backport all commits as part of the pull request 
-      instead of the suqashed one
+      Backport all commits found in the pull request.
+      The default behavior is to only backport the first commit that was merged in the base branch.
+    required: false
+  auto-no-squash:
+    description: >
+      If the pull request was merged or is open, backport all commits.
+      If the pull request commits were squashed, backport the squashed commit.
     required: false
-    default: "false"
   strategy:
     description: Cherry-pick merge strategy
     required: false