Add new GitHub workflow to sync branches through PR

This PR includes a new GitHub workflow to sync two branches through a PR (reference). The user must provide source and target branches as inputs.

Some notes:

The workflow will fail if any of the inputs are invalid.
The workflow won't create a PR if there are no changes to merge.
The workflow will create a PR even if there are conflicts. In this case, someone must send the fixes to the sync branch in a new PR. Once the fix PR is merged, the sync PR will be fixed as well.
The repository must have write permissions for workflows and also allow workflows to create PRs. I believe this repository already has these settings on.
Ideally, tests should be enough to guarantee that the PR is ready to be merged. Depending on the changes, we can decide to checkout the sync branch locally and perform some sanity checks.
Currently, this workflow is manually triggered but we can set this up to run on a schedule for particular branches, if it makes sense.

Description

I added this workflow in a new repository and tested different scenarios.

[x] The commits are squashed in a cohesive manner and have meaningful messages.
[x] Testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious).
[x] The developer has manually tested the changes and verified that the changes work