Suggestion: Let SqlServer source extension use a file for QueryText

[stefanedwards]

I propose that the SqlServer extension can point to a file to obtain the SQL statement.

This will improve the readability greatly for larger/multiline/complex SQL statements.

Essentially adding this to the SqlServerSource settings:

{
    "ConnectionString": "",
    "QueryText": "", // required if FilePath not set.
    "FilePath": ""   // required if QueryText not set.
}

I have implemented the solution, and it is ready for a pull request.

main...stefanedwards:data-migration-desktop-tool:add-query-file

Should the solution perhaps implement Cosmos.DataTransfer.Common.FileDataSource.ReadSourceAsync instead of directly reading the file?