Generalize `RowKeyType::clear()`

[dbolduc]

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.