[Oracle] Use QgsTask to retrieve available tables so we can properly cancel it

[troopa81]

Fixes #40001 : Use QgsTask instead of Qthread so we can properly cancel it from QgsTaskManagerWidget and when quitting the application.

For the record, createChildren is called inside a thread which is destroyed at the end of the function, so we need to move the task to the main thread so the sent signals/events have a QEventLoop running to process them.

[nyalldawson]

This is a GREAT move! I'd love to see the other providers doing this too at some stage.

[troopa81]

@nyalldawson Everything is green, is it OK to merge?

[troopa81]

@nyalldawson gentle ping, is it OK to merge?

[github-actions]

The QGIS project highly values your contribution and would love to see this work merged! Unfortunately this PR has not had any activity in the last 14 days and is being automatically marked as "stale". If you think this pull request should be merged, please check

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[qgis-bot]

The backport to queued_ltr_backports failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-queued_ltr_backports queued_ltr_backports
# Navigate to the new working tree
cd .worktrees/backport-queued_ltr_backports
# Create a new branch
git switch --create backport-43587-to-queued_ltr_backports
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 136381c8e45397d3fb118aa6491ea8f200fab12a
# Push it to GitHub
git push --set-upstream origin backport-43587-to-queued_ltr_backports
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-queued_ltr_backports

Then, create a pull request where the base branch is queued_ltr_backports and the compare/head branch is backport-43587-to-queued_ltr_backports.