feat: graceful kill for the bridge export and import functionalities

[a-hilaly]

for now interrupting git-bug while exporting or importing issues can damage local or remote and cause problems if when we re-execute these commands.
To avoid this we can implement a global context to gracefully shutdown these functions ! for this we need:

• To use a global context for the bridge importer and exporter
• use the context to cancel http requests
• Add cancel functions to Interrupt handler
• Commit operations before exiting the functions (SetMetadataOperation in the exporter for example `)

(this bug was created using the new exporter)

[MichaelMure]

(this bug was created using the new exporter)

🎉 🎉 🎉 🎉 🎉 🎉 🎉

great work !