Add compare operators to nostd::string_view

[maxgolov]

This change adds missing methods that exist on standard std::string_view , allowing to use nostd::string_view as key in std::map and other containers.

[codecov]

Codecov Report

Merging #124 into master will increase coverage by 0.04%.
The diff coverage is 96.77%.

@@            Coverage Diff             @@
##           master     #124      +/-   ##
==========================================
+ Coverage   93.45%   93.50%   +0.04%     
==========================================
  Files          71       71              
  Lines        1697     1724      +27     
==========================================
+ Hits         1586     1612      +26     
- Misses        111      112       +1     

Impacted Files	Coverage Δ
api/include/opentelemetry/nostd/string_view.h	91.89% <93.33%> (-0.42%)	⬇️
api/test/nostd/string_view_test.cc	100.00% <100.00%> (ø)

[g-easy]

Please also api/test/nostd/string_view_test.cc

[maxgolov]

Please also api/test/nostd/string_view_test.cc

Yes, I will add the tests.

[kmanghat]

Could we also add hash functions for nostd::string_view that make it usable with unordered_map and unordered_set? I can do this in a separate PR or it could be added in this one.

[maxgolov]

Could we also add hash functions for nostd::string_view

Are these part of standard std::string_view ? I'd like to ensure that our implementations of nostd classes are as close to standard library as possible. Why? Because that keeps an option on a table to substitute one for another in a neat way. I was planning to submit a separate PR that validates our nostd classes behavior side-by-side with Standard Library, C++17, C++20, as well as gsl::span. Abseil is another good candidate. Comparing just the bare minimum of classes we implement, and the methods we use in SDK.

[maxgolov]

Do you mean something like nostd::hash<nostd::string_view> compatible with std::hash ?

[kmanghat]

Yeah it looks like it is available in c++ 20, yeah I was thinking of something like that. You could also modify std::hash that way you don't have to do something like this std::unordered_set<nostd::string_view, nostd::MyHash> set

[reyang]

LGTM. Please rebase.