feat: Add basic session support

[Swatinem]

This adds release health sessions to the SDK, with some things that require special attention:

• sessions live on the scope.
• sessions are forward-inherited, which means that creating a new Hub from another one, and pushing scopes will use the exact same session.
• however, sessions are not backward-inherited, which means that starting a new session will not propagate backwards to any parent scopes/hubs.

So far, intermediate session updates are sent along with normal events, however session end updates are sent on their own and not yet batched together. This means that request-mode sessions are not yet as efficient as we would like them to be.