Add method to get the installation ID

### Problem Statement

In our App we have a button which allows the user to report issues.
For that, we would love to add some optional information like the installation ID from Sentry, so we can use that to analyse the user issues.
Right now I have no way to consistently get the information.

I tried setting a User manually, without any ID, which should lead to using the installation ID, which I should then be able to read out from the current scope, but that is not working reliably.

### Solution Brainstorm

So what I would love to have:

Sentry.getInstallationId() which would return the installation DI or Sentry.getUser(), which would return a default User, with the installation ID as user id.