FR: Allow specifying JVM flags for Java-based emulators

### [REQUIRED] Environment info

**firebase-tools:** 8.6.0

**Platform:** Windows 10, Chrome 84.0.4147.105

### [REQUIRED] Test case

I have no instructions to reproduce this issue, but I can describe the circumstances of it occurring

I have a large ruleset (around 60Ko) and many tests (200+) covering most of my ruleset.

When trying to visualize the report at http://localhost:8080/emulator/v1/projects/my-firebase-project:ruleCoverage I will get an error "sometimes", the only solution is then to restart the emulator and hope it will work the next time.

It seems that the report generation in Chrome is exhausting the capabilities of my laptop...

### [REQUIRED] Steps to reproduce

See above

### [REQUIRED] Expected behavior

Report should be generated

### [REQUIRED] Actual behavior

http://localhost:8080/emulator/v1/projects/my-firebase-project:ruleCoverage
{"error":{"code":500,"message":"Internal","status":"INTERNAL"}}

firestore-debug.log
Aug 04, 2020 9:52:10 AM io.gapi.emulators.netty.HttpHandler$1 onError
INFO: Exception when handling request: Java heap space

How would I increase the "Java heap space" for the emulator?