Quickly compute downstream dependency count per target

### Description of the feature request:

`minrank` and `maxrank` can be calculated for each target through a single command:
```
bazel query //... --output=minrank
bazel query //... --output=maxrank
```

However, calculating the downstream dependency count for each target, requires calling bazel many times:
```
bazel query 'rdeps(//..., )' | wc -l
bazel query 'rdeps(//..., )' | wc -l
bazel query 'rdeps(//..., )' | wc -l
...
```

Can it be done more efficiently?
Could a new `--output` type solve this?

I'm wondering what others think, and if there is a faster way to get these counts.
Since bazel is doing a topological sort, I'm hoping these counts could be exposed more efficiently.
My current approach is to dump the analysis cache, and run my own graph traversal to get these numbers.

### Which category does this issue belong to?

CLI, Performance

### What underlying problem are you trying to solve with this feature?

Get the downstream dependency count for every target as fast and efficiently as possible.

### Which operating system are you running Bazel on?

Linux

### What is the output of `bazel info release`?

release 6.2.0

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

_No response_

### What's the output of `git remote get-url origin; git rev-parse master; git rev-parse HEAD` ?

```text
Not relevant.
```

### Have you found anything relevant by searching the web?

No

### Any other information, logs, or outputs that you want to share?

Not relevant.