Add option maxCacheSize

### Description

See https://github.com/getsentry/team-mobile/issues/163.

SDKs with offline caching have the option `maxCacheItems` to specify the maximum number of envelopes to keep in the offline cache. Envelopes can significantly vary in size depending on which features our customers use. They can range from a couple of KBs up to MBs if, for example, screenshots are enabled.

As a mobile engineer, you mostly care about how much space the offline cache of Sentry SDKs will use on your user's devices. Therefore, we could add an option called `maxCacheSize` specifying the maximum size of bytes the offline cache can use until it has to delete envelopes. `maxCacheSize` and `maxCacheItems` should work together. Whenever the SDK stores an envelope to disk, it has to check if either `maxCacheSize` or `maxCacheItems` is reached, and if so, delete the oldest envelope.

What the default value for `maxCacheSize` should be is still being discussed.