[Feature] The `cache-max-entry-count` working off percentages makes it difficult to setup multiple servers

### Motivation

```
--cache-max-entry-count CACHE_MAX_ENTRY_COUNT
The percentage of free gpu memory occupied by the k/v cache, excluding weights . Default: 0.8. Type: float
```

I have a DGX machine, and I want to run multiple models on it. If I launched multiple servers concurrently (with tp=8), it would be ambiguous how much memory they would actually take up. By that, consider what would happen if I spun up a 1gb model with this set to 0.5, and then spun up a 100gb model with this set to 0.5.

The 1gb model would get half of the 640gb of memory that the machine has, which is 320gb, but the large model would only allocate 110gb for it's kv cache.

If I lauched them in the opposite order, the large machine would get 270gb, and the smaller one would get ~135gb.

For that reason, it'd be more convenient to be able to put in cache sizes in absolute amounts.

### Related resources

_No response_

### Additional context

_No response_