feat(cirrus): Set glean max event buffer value

[yashikakhurana]

Because

• When setting up glean sdk on cirrus side, we set the maxEvents to 500, which means it waits on the client side to gather all 500 events, and once it gets 500 then it sends the batch of events. Implementing applications should be able to adjust the value that works best for them.

This commit

• Add the option to set no. of events to store in buffer through .env, by default it uses 10 now

Fixes #9534

[jaredlockhart]

LGTM thnx @yashikakhurana !