Add new backup database dialog to MSSQL extension

[corivera]

This dialog uses the object management framework like our other admin dialogs, but re-uses the same STS calls as our existing backup experience. However, this dialog currently only supports backing up to a local file. I'll be adding URL support in subsequent PRs. We'll keep the current backup dialog around until the new dialog has the same feature parity and passes accessibility testing.

Closes #24588

Screenshots:

With options expanded:

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Pull Request Test Coverage Report for Build 6792116662

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• 0 of 0 changed or added relevant lines in 0 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.004%) to 41.577%

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Files with Coverage Reduction	New Missed Lines	%
extensions/sql-database-projects/src/common/httpClient.ts	1	66.18%
extensions/sql-database-projects/src/dialogs/publishDatabaseDialog.ts	2	52.03%

Totals
Change from base Build 6780078081:	0.004%
Covered Lines:	30789
Relevant Lines:	69355

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💛 - Coveralls

[corivera]

The existing backup & restore STS methods do their operations in the background, so we don't get a script back from the method call. This change is so we can return immediately without waiting on results.