Saving/loading SQL queries from Execute SQL/Update SQL dialogs

[nyalldawson]

This adds support for saving and loading SQL queries to a .sql text file to the Execute SQL dialog and Update SQL dialogs.

Effectively, it ports this functionality from the DB Manager plugin over to the core browser-based database connection facilities.

The UX has been designed to mimic the same functionality from other standard parts of QGIS, eg the Processing Script Editor. Toolbar actions are used accordingly, instead of the old text button approach used in DB Manager. To accompany the new toolbar I've also added standard actions for operations like copy/paste/undo/redo, again mimicing the Processing Script Editor dialog UI

Sponsored by City of Canning

[qgis-bot]

@nyalldawson

This pull request has been tagged for the changelog.

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[qgis-bot]

@nyalldawson
This pull request has been tagged as requiring documentation.

A documentation ticket will be opened at https://github.com/qgis/QGIS-Documentation when this PR is merged.

Please update the description (not the comments) with helpful description and screenshot to help the work from documentors.
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[github-actions]

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[qgis-bot]

@nyalldawson
A documentation ticket has been opened at qgis/QGIS-Documentation#9667
It is your responsibility to visit this ticket and add as much detail as possible for the documentation team to correctly document this change.
Thank you!