Improve release macro/auto release detection

### Problem Statement

The `release_name` macro defaults to the cargo package name + version. This is not useful in a server application, where the version might be always `0.1.0`. There, the user needs to specify the release manually. It would make sense if the SDK does this instead.

### Solution Brainstorm

It would probably be better to use the Git commit hash, as done for instance by the Python SDK: https://github.com/getsentry/sentry-python/blob/7d4ac90be7a485cab70e37ccc831ca86b8f9c30d/sentry_sdk/utils.py#L148.
Arguably this could be done automatically under the hood, instead of having the user need to use the `release_name` macro. Right now we only read `SENTRY_RELEASE` under the hood and do the detection in the macro.

Start with the `release_name` macro and the code that currently reads `SENTRY_RELEASE`; compare the proposed Git commit detection with the linked Python SDK implementation. Determine how automatic detection should interact with the macro and environment variable, then verify that server applications receive a useful release without manual configuration.

Written by the indexing model from the issue text.