[MD] Check data source name existence before creating

[zengyan-amazon]

We want to avoid having duplicated data source name, which may confuse user and cause bad experience. So when creating a new data source, we need to check if there is already a data source with the same name was already created, and give a warning to user to choose a different data source name.

[mpabba3003]

This is done from UI only. There will be an error that blocks user from creating a data source with duplicate title.