add explain and join snippets

[morganchen12]

No description provided.

[gemini-code-assist]

Code Review

This pull request updates the Xcode project to use remote package references for the Firebase iOS SDK instead of local ones, and adds numerous pipeline snippets to PipelineSnippets.swift demonstrating advanced Firestore features like joins, search, and aggregations. The review feedback highlights duplicate and dangling package references in the project file that should be cleaned up, and points out that several newly added snippet methods assign the query execution results to an unused results variable, which will trigger Swift compiler warnings. It is recommended to print these results to resolve the warnings and maintain consistency.