Add Client::Post with both content provider and receiver

[clarkok]

Add most Post overloads for Client, with both content provider and content receiver.

• Result Post(const std::string &path, size_t content_length, ContentProvider content_provider, const std::string &content_type, ContentReceiver content_receiver, UploadProgress progress = nullptr)
• Result Post(const std::string &path, ContentProviderWithoutLength content_provider, const std::string &content_type, ContentReceiver content_receiver, UploadProgress progress = nullptr)
• Result Post(const std::string &path, const Headers &headers, size_t content_length, ContentProvider content_provider, const std::string &content_type, ContentReceiver content_receiver, DownloadProgress progress = nullptr)
• Result Post(const std::string &path, const Headers &headers, ContentProviderWithoutLength content_provider, const std::string &content_type, ContentReceiver content_receiver, DownloadProgress progress = nullptr)

[yhirose]

@clarkok thanks for the pull request. But I am not going to approve it since it create another two POST specific implementations. Could you please rename send_with_content_provider to send_with_content_provider_and_receiver, and add your receiver support to it? Also could you add the same methods to PUT and PATCH for consistency? Then, I'll start review your pull request. Thanks!