Add source index build step

[davidfowl]

See dotnet/runtime#46823 for more details. This allows EF core to show up in https://source.dot.net/

cc @alexperovich
cc @bricelam

[dougbu]

@bricelam this change looks straightforward and is something we should do together w/ dotnet/aspnetcore#33534. @dotnet/aspnet-build should make short work of it.

[bricelam]

enableRichCodeNavigation and source-index-stage1.yml?? Doesn't seem very DRY...

[alexperovich]

To simplify this change, if the repo uses eng/common/templates/jobs/jobs.yml, you can just set the "enableSourceIndex" parameter there. If the code to be indexed gets built by default with eng/common/build.ps1, then you can stop there. Otherwise the sourceIndexBuildCommand value in the sourceIndexParams parameter will need to be set to the command that will produce a binlog containing builds of all the projects to be indexed.

[AndriySvyryd]

This is done from the EF side. Opened dotnet/source-indexer#116 to track the remaining work.