Add podspec publishing workflow

[pblazej]

Publishes LiveKitWebRTC.podspec to livekit/podspecs when a release is published (plus manual dispatch), via livekit/publish-cocoapod-action. Same workflow as livekit/client-sdk-swift#1125, only podspec: differs.

• Xcode pinned to 26.5, matching client-sdk-swift's cocoapods-lint.yaml. Xcode 27 rejects deployment targets below iOS 15 / macOS 12 as an error, so latest-stable would break validation of this pod once runners move to it.
• Action pinned to Keep dry-run off the remote publish-cocoapod-action#1 (keeps dry-run off the remote). Re-pin to main once that merges.
• Trigger timing: the release here is the draft opened by the hosting PR, so merge the PR first, then publish the release (as that PR already says). Publishing first puts the tag on the old main; the run then sees an already-published version and fails safely on --no-overwrite.

Specific to this pod

• Validates on all four declared platforms, including tvOS 17 and visionOS 26. The macos-26 image ships those simulator runtimes; nothing extra to install.
• :http source points at the release asset, which only a published release serves. That matches the release: published trigger.

Tested

Dry run on a real runner (via a temporary pull_request trigger, since dropped). It validated 150.7871.03 on iOS 13 / macOS 10.15 / tvOS 17 / visionOS 26 and committed [Update] LiveKitWebRTC (150.7871.03) locally; push and PR steps were skipped.

After merging

150.7871.03 (released Oct 5) isn't in livekit/podspecs yet. Publish it by dispatching this workflow with ref: 150.7871.03, dry_run: false.