Enable grpc/swift fetcher back

[edigaryev]

In #1695, grpc/swift fetcher was disabled.

However, it seems that:

• the gRPC Swift version 2 now lives in a separate github.com/grpc/grpc-swift-2 repository
• which shouldn't cause any conflicts for github.com/grpc/grpc-swift, which is now at 1.27.1

We need this for github.com/cirruslabs/tart, where:

• we're currently limited to version 1.24.2 of gRPC Swift
• ...but uses version 1.27.1
• ...which causes compilation failure

[CLAassistant]

All committers have signed the CLA.

[pkwarren]

Thank you for the PR. I will merge however we'll need a slight tweak to the version detection code to support pinning this plugin to v1.x tags.

[pkwarren]

The grpc/swift plugin is updated to v1.27.1 and should be available shortly on the BSR.

[edigaryev]

The grpc/swift plugin is updated to v1.27.1 and should be available shortly on the BSR.

Awesome, it's already already available on the BSR, thank you so much!