feat(bigtable): add Clear(std::string&) helper for RowKeyType compatibility

Adds a Clear(std::string&) function that calls std::string::clear()
in OSS builds. This unifies the interface for clearing row keys
between OSS (std::string) and Google internal (absl::Cord), where
an overload will call absl::Cord::Clear() instead of deprecated clear().

Signed-off-by: Dronaraj Gyawali <dronarajgyawali@gmail.com>

Author: drona-gyawali

google/cloud/bigtable/internal/google_bytes_traits.h

@@ -38,6 +38,9 @@ namespace internal {
 /// Return true if the row key is empty.
 inline bool IsEmptyRowKey(std::string const& key) { return key.empty(); }
 
+/// Clear the row key.
+inline void Clear(std::string& key) { key.clear(); }
+
 /// Return true if the row key is empty.
 inline bool IsEmptyRowKey(char const* key) { return std::string{} == key; }