I'm in a somewhat situation, except that I have already begun HRT and plan to find a job as part of a major relocation. I do not anticipate coming out to the public for at least a year from now, perhaps longer depending on how things work out (and assuming HRT gives slow, subtle changes over that time, though I wouldn't complain in the long term if it goes too fast and strong and gets in the way of my plans...)
Anyway, I do not plan to discuss this with potential employers. It is none of their business whatsoever, and gender identity is protected from discrimination anywhere I would consider settling. That reinforces its not being their business. However, legal protections from discrimination are of very limited practical value in many cases. My plan to combat this a bit is to delay any workplace transition until I've been there long enough to establish myself as a value to the employer, and if possible, to get a positive performance review or equivalent on record. That at least makes it harder for them to claim I was a poor employee and they'd have to try a little harder to mask their reasons if they try to fire me.