OPENTHREAD_CONFIG_NET_DIAG_VENDOR_INFO_SET_API_ENABLE macro

@def OPENTHREAD_CONFIG_NET_DIAG_VENDOR_INFO_SET_API_ENABLE Define as 1 to add APIs to allow Vendor Name, Model, SW Version to change at run-time. It is recommended that Vendor Name, Model, and SW Version are set at build time using the OpenThread configurations `OPENTHREAD_CONFIG_NET_DIAG_VENDOR_*`. This way they are treated as constants and won't consume RAM. However, for situations where the OpenThread stack is integrated as a library into different projects/products, this config can be used to add API to change Vendor Name, Model, and SW Version at run-time. In this case, the strings in `OPENTHREAD_CONFIG_NET_DIAG_VENDOR_*` are treated as the default values (used when OT stack is initialized). Enabled by default for reference devices, when `OPENTHREAD_CONFIG_REFERENCE_DEVICE_ENABLE` is defined.