[NSD] Add new GetTxtRecords api (#3353)
* [NSD] Add new GetTxtRecords api
Related PR: https://github.com/Samsung/TizenFX/pull/2418
* Update src/Tizen.Network.Nsd/Tizen.Network.Nsd/DnssdService.cs
Co-authored-by: WonYoung Choi <wy80.choi@samsung.com>
* Update src/Tizen.Network.Nsd/Tizen.Network.Nsd/DnssdService.cs
Co-authored-by: WonYoung Choi <wy80.choi@samsung.com>
* Fix build error
Error: Tizen.Network.Nsd/DnssdService.cs(273,20): error CS0128: A local variable or function named 'txtValue' is already defined in this scope
Co-authored-by: WonYoung Choi <wy80.choi@samsung.com>