Automate publishing podspecs to CocoaPods (#313)

Resolves #311 by
adding a job which runs when new release tags are created and
automatically publishes new releases to CocoaPods. I also updated the
documentation to reflect the new release process.

A sample run can be seen here (against this PR, not a release tag):
https://github.com/connectrpc/connect-swift/actions/runs/11345378532/job/31552734926?pr=313

---------

Signed-off-by: Michael Rebello <[email protected]>

Author: rebello95

.github/CONTRIBUTING.md

@@ -173,23 +173,12 @@ To update dependencies such as `SwiftProtobuf` in this repository:
 
 Releases should be tagged in `x.y.z` SemVer format.
 
-1. Create a new GitHub release.
-2. Update both [`Connect-Swift.podspec`](../Connect-Swift.podspec) and
+1. Update both [`Connect-Swift.podspec`](../Connect-Swift.podspec) and
    [`Connect-Swift-Mocks.podspec`](../Connect-Swift-Mocks.podspec) to reflect
-   the newly tagged version.
-3. Run `cd Examples/ElizaCocoaPodsApp && pod update` to update the example CocoaPods app.
-4. Submit a PR with these changes.
-5. Push both specs to CocoaPods:
-
-```sh
-pod trunk push Connect-Swift.podspec
-pod repo update
-pod trunk push Connect-Swift-Mocks.podspec
-```
-
-Note: If pushing the mocks podspec fails because CocoaPods cannot find the new
-`Connect-Swift` podspec in the specs repo, you may have to wait ~30 min
-for it to populate before trying again.
+   the upcoming version on `main`. In the same PR, update the example CocoaPods
+   app by running `cd Examples/ElizaCocoaPodsApp && pod install`.
+2. Create a new GitHub release/tag after merging these changes.
+3. CI will automatically push the updated specs to CocoaPods in its [release job](./workflows/release.yml).
 
 [dco]: https://developercertificate.org
 [commit-message]: http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html

.github/workflows/release.yml

@@ -36,3 +36,17 @@ jobs:
           append_body: true
           files: |
             ./.tmp/bin/artifacts/*
+  publish-podspecs:
+    runs-on: macos-14
+    steps:
+      - uses: actions/checkout@v4
+      - name: Select Xcode version
+        # https://github.com/actions/runner-images/blob/main/images/macos/macos-14-arm64-Readme.md#xcode
+        run: sudo xcode-select --switch /Applications/Xcode_16.app
+      - name: Publish podspecs to CocoaPods
+        env:
+          COCOAPODS_TRUNK_TOKEN: ${{ secrets.COCOAPODS_TRUNK_TOKEN }}
+        # Note that --synchronous is used since Mocks depends on the primary spec.
+        run: |
+          pod trunk push Connect-Swift.podspec --allow-warnings --synchronous
+          pod trunk push Connect-Swift-Mocks.podspec --allow-warnings --synchronous