Someone who specializes in tea is called simply a tea specialist.
A tea specialist is someone who has been educated in tea’s history and making tea, categories or types of tea and the methods of tea production and processing.
If they are working in a restaurant, it is called a tea sommelier. This term would be equivalent to sommelier (or more specifically wine sommelier) and barista. For example, there are certificate programs to become a tea sommelier.
n. a restaurant worker employed as an expert on teas.
Note: Barista is someone who makes and serves coffee (usually espresso-based coffee drinks) but its meaning has expanded and they might be called a coffee sommelier also. Today, not all baristas are coffee experts but more of them are becoming experts. For example, Nespresso started this coffee sommelier program.
Apparently, sommelier is a versatile term that can be applied to many fields.
The terms beer sommelier and sake sommelier are sometimes used for beer and sake. In Japan, sommelier describes not only wine experts but other fields of expertise, for instance music sommelier,or vegetable sommelier which is a certification delivered by the Japan Vegetable Sommelier Association (日本野菜ソムリエ協会).