spannertest: add support for PROTO type

**Is your feature request related to a problem? Please describe.**
The `PROTO` type was [recently released](https://cloud.google.com/spanner/docs/reference/standard-sql/protocol-buffers), and we're eager to use it. We need it to be supported in `spannertest` for our unit test suite to not fail when encountering a `PROTO` type.

**Describe the solution you'd like**
`spannertest` should support the `PROTO` type, in terms of schema definitions, indexes, read and write operations via Spanner APIs, as well as loading a proto bundle with the proto descriptors.

**Describe alternatives you've considered**
* Using BYTES instead, which doesn't provide the powerful capabilities enabled by this new feature.

**Additional context**
NA