[csharp-netcore] Improved Multimap and ClientUtils implementation

[shibayan]

PR detail

• Improvement Multimap<K,V>
  • Do not use ConcurrentDictionary<K,V> because it is not operated from multiple threads
  • Adding overload methods Add(Multimap<K,V>)
  • Improved API implementation complexity
• Improved runtime type checking
  • Use pattern matching
• Changed to an implementation that considers memory usage efficiency
  • Improved stream handling

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@mandrean @jimschubert @frankyjuang

[wing328]

LGTM. If no further feedback, I'll merge the PR tomorrow.

[shibayan]

The conflict has been resolved.

[wing328]

@shibayan do you mind resolving the merge conflicts again? I may have merged other PRs first resulting in the merge conflicts.