Allow to set custom index.html and assets for teleop UI

[ynaka81]

I want to support some extra use cases where, for instance, to teleoperate robots with hand tracking with Meta Quest 3 or to stream the video mounted on the robot.
This PR gives the ability to set an arbitrary frontend implementation. This makes developers use their own frontend implementation while keeping the current basic implementation.

[ynaka81]

@lukicdarkoo @MarijaGolubovic
Sorry to bother you, but I'm just checking in on this PR.
I believe this change is safe and won't break any existing features.

Is there anything that I can do to help with the review?
If you’re busy, no rush, but I’d appreciate it if you could take a look when you have a moment.

[lukicdarkoo]

Hi @ynaka81 , looks good, thank you!

Do you have an example with the hand tracking?

[ynaka81]

@lukicdarkoo
Thank you for your review! I'd appreciate it if you merge and release this PR as a new version, because I don't have the permission to merge this PR.

Regarding the example,
Yes, I have it, but it's pretty much a prototype. I can update you once finalizing the implementation.

[lukicdarkoo]

Regarding the example,
Yes, I have it, but it's pretty much a prototype. I can update you once finalizing the implementation.

@ynaka81 keep me posted, I would love to see it!