as the key is not always nul terminated, it'd be handy to use
a `std::string_view` when a key is expected.
before this change, we would have FTBFS if the key is a `string_view`
when looking up a node in a map:
```
/usr/include/yaml-cpp/node/detail/impl.h: In instantiation of ‘bool YAML::detail::node::equals(const T&, YAML::detail::shared_memory_holder) [with T = std::basic_string_view<char>; YAML::detail::shared_memory_holder = std::shared_ptr<YAML::detail::memory_holder>]’:
/usr/include/yaml-cpp/node/detail/impl.h:122:26: required from ‘YAML::detail::node* YAML::detail::node_data::get(const Key&, YAML::detail::shared_memory_holder) const [with Key = std::basic_string_view<char>; YAML::detail::shared_memory_holder = std::shared_ptr<YAML::detail::memory_holder>]’
/usr/include/yaml-cpp/node/detail/node_ref.h:64:55: required from ‘YAML::detail::node* YAML::detail::node_ref::get(const Key&, YAML::detail::shared_memory_holder) const [with Key = std::basic_string_view<char>; YAML::detail::shared_memory_holder = std::shared_ptr<YAML::detail::memory_holder>]’
/usr/include/yaml-cpp/node/detail/node.h:125:53: required from ‘YAML::detail::node* YAML::detail::node::get(const Key&, YAML::detail::shared_memory_holder) const [with Key = std::basic_string_view<char>; YAML::detail::shared_memory_holder = std::shared_ptr<YAML::detail::memory_holder>]’
/usr/include/yaml-cpp/node/impl.h:392:71: required from ‘const YAML::Node YAML::Node::operator[](const Key&) const [with Key = std::basic_string_view<char>]’
../../src/config/base_config.cc:20:23: required from here
/usr/include/yaml-cpp/node/detail/impl.h:93:25: error: incomplete type ‘YAML::convert<std::basic_string_view<char> >’ used in nested name specifier
93 | if (convert<T>::decode(Node(*this, pMemory), lhs)) {
| ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
```
after this change, a full specialization for `convert<std::string_view`
is added so that we can lookup a node using key of `std::string_view`, like
```c++
string_view key = "yet_another_key";
auto value = node[key];
```
tested with
```console
$ cmake -B build -D CMAKE_CXX_STANDARD=20 -D YAML_CPP_BUILD_TESTS=ON -G Ninja
$ cmake --build build yaml-cpp-tests
$ ctest --test-dir build --output-on-failure
```
Signed-off-by: Kefu Chai <tchaikov@gmail.com>
3ff13a243d