Use standard media types as specified by RFC8040 and RFC8072

There are a few issues with the media types we are producing:

• we are using application/yang.patch instead of application/yang-patch
• we are using application/yang.patch-status instead of application/yang-data
• we allow producing application/yang-data (without encoding)
• we produce application/yang-data instead of yang-data+xml
• we produce application/xrd+json instead of application/json

Make sure we produce correct types.