Allow volume levels of individual voice call members to be configured.

### Your use case

#### What would you like to do?

I'd like individual voice call volumes, similar to how Discord handles it, in which member volume can be raised or lowered anywhere between 0% and 200% (though that upper boundary has no reason to be so low), and remembered between calls, and between 1:1 and n-member calls.

#### Why would you like to do it?

Loudness normalization. Some people have quiet microphones, and some people have loud microphones. Out of personal experience, something I mentioned in vector-im/element-web#2027, this is a fairly essential feature for voice calls, and personally makes it difficult to justify using Element over another program such as Discord, for myself and for my friends.

#### How would you like to achieve it?

As mentioned in the first paragraph, similar to how Discord does it, with a slider allowing you to raise or lower a member's effective volume between 0% and 200%, defaulting at 100%

### Have you considered any alternatives?

I'm aware that in n-member calls that it uses Jitsi, which does allow this, but this doesn't help in one-on-one calls. Technically speaking, it is possible to change the volume of the entire program for each 1:1 call, but this is limited by the fact that it cannot boost volume above 100%, and is also an extremely poor user experience depending on how many different people you call.

### Additional context

_No response_