Add build workflow

[oleole39]

Hello,

This PR adds a build workflow that is to be triggered manually in the Actions tab.
Here is a summary of what it does:

• Takes the app version number defined in pubspec.yaml
• Creates a Github tag based on the version number
• For each platform:
  • Install Flutter and platform-specific dependencies
  • Build the app
  • Upload built artifiacts to a released (named after the version number)
• Publish the release. Note that the PR currently uses dry-run: true parameter for test purposes, and that it will have to be toggled to false so that the release actually gets created.

A (not dry-) run log can be found there: https://github.com/oleole39/netshare/actions/runs/26201793948
Together with its corresponding published release: https://github.com/oleole39/netshare/releases/tag/2.2.0

The workflow seems to work fine, but there are issues in the build process. I tried to follow instructions in the repo's README, but those might be outdated. Here are platform-related comments (more info can be found in the log):

• Android: build fails, likely due to missing keyfile for signed APK build process. Introducing a build for an unsigned APK could be an idea.
• Windows: build succeeds. Untested.
• Linux: build succeeds. The .deb file can be installed. The installed app can be launched, however it just opens a black window with nothing in it (tested on Linux Mint 22.2).
• MacOS: build fails, for some reason.
• iOS: build fails (although the Github runner says it succeeded) - no file is built and therefore no artifiact uploaded.

PS: A tricky point with Github FYI just in case - workflows appear in the Actions tab only when added to the master branch (or at least a first one, later versions can be run from other branches provided they have the same name than the one in the master branch).