Add build verification using GitHub Actions

Since this repository is used for snippets that are included in docs, the code here should have some automated verification that it compiles. Travis CI would be ideal as it's consistent with other Firebase Github projects.

Unit testing is not required but is a nice-to-have.

Start by inspecting the repository's existing C++ build entry points and how the snippets are organized. Define completion as an automated GitHub Actions check that verifies the snippets compile; unit tests are optional, and the issue also mentions Travis CI as an alternative to reconcile.

Written by the indexing model from the issue text.