Generalize `RowKeyType::clear()`

`bigtable::RowKeyType` is a `std::string` in OSS-land, and an `absl::Cord` in Google-land.

`absl::Cord::clear()` is deprecated, in favor of `absl::Cord::Clear()`. `std::string::Clear()` is not a thing.

We need to add a new `Clear(RowKeyType&)` function to this file:

https://github.com/googleapis/google-cloud-cpp/blob/main/google/cloud/bigtable/internal/google_bytes_traits.h

which calls `std::string::clear()` in OSS-land, and `absl::Cord::Clear()` in Google-land.

This unblocks a LSC in Google-land.