Allow use of an existing SQL Connection

[droyad]

Pick of #576 into this repo and update to match latest code base

This pull request adds support for passing an existing SqlConnection to the SqlConnectionManager and SqlServerExtensions.SqlDatabase methods, enabling more flexible connection management when upgrading SQL Server databases. This enhancement allows consumers to reuse existing connections, which can be useful for advanced scenarios such as connection pooling or custom authentication.

Support for existing SqlConnection:

• Added a new constructor to SqlConnectionManager that accepts an existing SqlConnection object, allowing direct management of externally-created connections. [1] [2]
• Added two new overloads to SqlServerExtensions.SqlDatabase:
  • One that accepts a SqlConnection (without schema), and
  • One that accepts a SqlConnection and a schema string, both returning an UpgradeEngineBuilder for SQL Server upgrades. [1] [2] [3]