Add grpc/swift-protobuf v2.1.2 (#2161)

Needed an update to the dependencies (related to #1941). For context,
the steps were (from the plugin's directory):

```
$ git clone --depth 1 --branch 2.1.2 https://github.com/grpc/grpc-swift-protobuf --recursive
$ cd grpc-swift-protobuf
$ swift package resolve
$ mv Package.resolved ../
```

Probably a good reason to look to automate this now.

Fixes #2160.

Author: stefanvanburen

plugins/grpc/swift-protobuf/v2.1.2/.dockerignore

@@ -0,0 +1,3 @@
+*
+!Dockerfile
+!Package.resolved

plugins/grpc/swift-protobuf/v2.1.2/Dockerfile

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+# syntax=docker/dockerfile:1.19
+FROM swift:6.2.3-bookworm AS build
+
+RUN apt-get update \
+ && apt-get install -y unzip
+WORKDIR /app
+RUN git clone --depth 1 --branch 2.1.2 https://github.com/grpc/grpc-swift-protobuf --recursive
+WORKDIR /app/grpc-swift-protobuf
+COPY --link Package.resolved .
+RUN swift build -c release --static-swift-stdlib --product protoc-gen-grpc-swift-2 -Xlinker -s --force-resolved-versions
+
+FROM gcr.io/distroless/cc-debian12:latest@sha256:0c8eac8ea42a167255d03c3ba6dfad2989c15427ed93d16c53ef9706ea4691df AS base
+
+FROM scratch
+COPY --link --from=base / /
+COPY --link --from=build /app/grpc-swift-protobuf/.build/release/protoc-gen-grpc-swift-2 .
+USER nobody
+ENTRYPOINT [ "/protoc-gen-grpc-swift-2" ]

plugins/grpc/swift-protobuf/v2.1.2/Package.resolved

@@ -0,0 +1,31 @@
+version: v1
+name: buf.build/grpc/swift-protobuf
+plugin_version: v2.1.2
+source_url: https://github.com/grpc/grpc-swift-protobuf
+integration_guide_url: https://swiftpackageindex.com/grpc/grpc-swift-protobuf/2.1.2/documentation/grpcprotobuf
+description: Generates Swift client and server stubs for the gRPC framework.
+deps:
+  - plugin: buf.build/apple/swift:v1.33.3
+output_languages:
+  - swift
+registry:
+  swift:
+    deps:
+      - source: https://github.com/grpc/grpc-swift-protobuf.git
+        package: grpc-swift-protobuf
+        # https://github.com/grpc/grpc-swift-protobuf/blob/2.1.2/Package.swift#L22
+        products: [ GRPCProtobuf ]
+        version: 2.1.2
+        # https://github.com/grpc/grpc-swift-protobuf/blob/2.1.2/Package.swift#L65
+        swift_versions: [ ".v6" ]
+        # https://github.com/grpc/grpc-swift-protobuf/blob/2.1.2/Package.swift#L58
+        platforms:
+          macos: v15
+          ios: v18
+          tvos: v18
+          watchos: v11
+  opts:
+    - Visibility=Public
+    - FileNaming=PathToUnderscores
+spdx_license_id: Apache-2.0
+license_url: https://github.com/grpc/grpc-swift-protobuf/blob/2.1.2/LICENSE

plugins/grpc/swift-protobuf/v2.1.2/buf.plugin.yaml

@@ -0,0 +1 @@
+h1:t5ycbVWI4y9Z5KuSJqQ4bQPdpHrWwVbMjESFbawChEE=

tests/testdata/buf.build/grpc/swift-protobuf/v2.1.2/eliza/plugin.sum

@@ -0,0 +1 @@
+h1:icvJdhaYuOrR7ZCJZXU4y9CqvP1LJnJ+T7nvYk7SFuo=