Tools: Honor launchSettings.json

[bricelam]

The CLI added support for launchSettings.json in dotnet/cli#6617. Unfortunately, it only works with dotnet run and thus doesn't light up in our tooling.

If we want to honor launchSettings.json (particularly environment variables and command line arguments) in our tools, we'll have to look for and parse the file ourselves.

If we do this, we should also remove our existing logic that defaults the ASPNETCORE_ENVIRONMENT variable to Development.

[ajcvickers]

@danroth27 @DamianEdwards @Eilon @divega It would be great to have launchSettings.json be something that would work when using EF tooling. However, we're not going to try to hack this in by trying to parse the file in our own code. Do you guys see this as something we should push the CLI team to support? If so, we'll file an issue for it in dotnet/cli.

[ajcvickers]

@divega to file an issue on CLI side.

[divega]

We need a reusable component that dotnet run, VS and us can use.