Proposal: Unified Extension points for Imports

W.R.T to #1686

If we do get a Base SDK and/or a Managed SDK that's above .NET SDK, then I would propose to unify the Import Extensibility points they offer.

#### Base SDK props
- `Directory.Build.props`
- `..Before.props`

#### Any SDK props
- `CustomBeforeProps`
- ``
- `CustomAfterProps`

#### Base SDK props
- `..After.props`

#### Project File
- `CustomImportBeforeProject` / `CustomBefore` / `Custom.Project.props`
- ``
- `CustomImportAfterProject` / `CustomAfter` / `Custom.Project.targets`

#### Base SDK targets
- `..Before.targets`

#### Any SDK targets
- `CustomBeforeTargets`
- ``
- `CustomAfterTargets`

#### Base SDK targets
- `..After.targets`
- `Directory.Build.targets`

**NOTE**: `Custom Before/After` are user extensibility points. They are optional as they depend on the SDK implementers. [MSBuildSdks](/Microsoft/MSBuildSdks) already follows the `Custom` pattern I mentioned above.