Adds performance monitoring triggers to v2 alerts

[colerogers]

Adds Firebase Performance Monitoring triggers for performance threshold alerts to the v2 namespace.

// listen on all apps in a project
export const fn = onThresholdAlertPublished((event: PerformanceEvent<ThresholdAlertPayload>) => { … });

// listen on a specific app with a supplied appId
export const fn2 = onThresholdAlertPublished("1234567", (event: PerformanceEvent<ThresholdAlertPayload>) => { … });

// listen on a all apps with an options object
export const fn3 = onThresholdAlertPublished(
  {
    region: "us-central1",
  },
  (event: PerformanceEvent<ThresholdAlertPayload>) => { … }
);

// listen on a specific app with a supplied appId in the options object
export const fn4 = onThresholdAlertPublished(
  {
    appId: "1234567",
    region: "us-central1",
  },
  (event: PerformanceEvent<ThresholdAlertPayload>) => { … }
);

[TheIronDev]

LGTM

[taeold]

nit* I think we say "Cloud Function" instead of function in remote config. I think I prefer the former. wdyt?

[colerogers]

Oh good catch, but I think that remote config should actually be changed. Looking at all the other alert functions (& most other v2 functions) we say 'A function...'.

I remember @egilmorez giving us guidance here a while back, but not sure which way

[colerogers]

@taeold I just look through my old PR's and confirmed the recommended writing is "A function ...", I opened up #1232 to fix the other occurrences.