Initial commit for dotnet sln configuration cmd

[vrushalivartak]

As reported in #10359 support for custom configurations is missing currently in dotnet cli.
A new dotnet sln configuration command will enable to add, remove, rename, update the config or platform to a solution file.
This PR is for 'add' command, limited to adding config to a solution file.
dotnet sln [<SOLUTION_FILE>] configuration add -c|--config [-cf|--copyfrom ] [-up|--updateproj <y/n>]

PRs for, 1) 'add' command to add platform 2) 'remove', 'rename', 'update' commands, to follow after this PR is approved.

Original changes were submitted in #29139. @marcpopMSFT As suggested, the changes have been reworked to move the configuration command to be part of dotnet sln.

[vrushalivartak]

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