Use string_view for non-owning string references

Feature request

Currently, the API uses const std::string & for non-owning references to strings (e.g., topic names, node names). A more modern solution would be to use string_view. String constants can be declared as string_view in a header without initializing them in every translation unit. They are also constexpr, which can potentially open up some compile-time usage.

Feature description

The blocker for implementing this right now is that the rcl layer expects null-terminated strings. It's not possible to know if the byte following the string_view buffer is a null byte or simply uninitialized memory, so one would have to copy the data to a string anyway.

Right now, I don't see how this is viable without modifying the rcl API to accept string + length parameters. I thought I'd make the issue regardless, as from a pure C++ API standpoint, using string_views would perhaps be better.

Start by auditing the public C++ API uses of const std::string& for non-owning references and tracing where those values cross into the rcl layer. Determine whether the rcl API can support string-plus-length inputs without breaking callers. Done requires a concrete, agreed scope for string_view adoption or a documented resolution of the null-termination blocker.