Serialize to UTF-8 bytes with System.Text.Json.

[olegd-superoffice]

Serializing to a UTF-8 byte array is about 5-10% faster than using the string-based methods.

[ovska]

This can be further improved by using JsonContent so the payload is serialized directly to Http stream: JsonContent.Create(payload, options: _settings.Value). The downside is that this api seems to be available only from .NET 5 onwards, so a #if NET5_0_OR_GREATER preprocessor symbol might be required, which make the method more complex.
https://docs.microsoft.com/en-us/dotnet/api/system.net.http.json.jsoncontent.create?view=net-6.0

[olegd-superoffice]

This can be further improved by using JsonContent so the payload is serialized directly to Http stream: JsonContent.Create(payload, options: _settings.Value). The downside is that this api seems to be available only from .NET 5 onwards

@ovska I think this API is available for any supported .Net version, but it adds dependency on System.Net.Http.Json NuGet package. Could be worth another PR when/if this one is accepted.

[olegd-superoffice]

I updated this PR to use UTF-8 byte array serialization when producing application/x-www-form-urlencoded in generated client.

[RicoSuter]

Can you fix the conflicts, otherwise looks good.

[olegd-superoffice]

@RicoSuter done.