vulkan: store in GstVulkanVideoCapabilities encoder and decoder caps
The structure already stored the generic video capabilities and the specific codec capabilities both for encoding an decoding. The generic decoder capabilities weren't stored because it was only used internally in the decoder helper object. Nonetheless, for the encoder, the elements will need the generic encoder capabilities to configure the encoding. That's why it's required to expose it as part of GstVulkanVideoCapabilities. And the generic decoder is included for the sake of symmetry.