fix: set default max_spans to 1000

[JoshuaMoelans]

Fixes #1131

• set default for sentry_options_new
• think about other occurrences of SENTRY_SPANS_MAX

[JoshuaMoelans]

We have three of these snippets for transaction/span_start_child and span_finish.
Even if we set the options->max_spans at creating the options object, I'm not sure we can just remove these?

  size_t max_spans = SENTRY_SPANS_MAX;
  SENTRY_WITH_OPTIONS (options) {
      max_spans = options->max_spans;
  }

[supervacuus]

We have three of these snippets for transaction/span_start_child and span_finish. Even if we set the options->max_spans at creating the options object, I'm not sure we can just remove these?

The sole reason why those exist is that the initial implementer didn't want to scope the remaining code in SENTRY_WITH_OPTIONS. However, they had to declare it outside so that it was accessible, and that naturally led to potential uninitialized use, which led them to initialize it to the most sensible value. 😅

I would leave them in there. They don't hurt anyone. If you want to get rid of them, you will have to put the entire dependent code inside the SENTRY_WITH_OPTIONS, which doesn't add a lot but might visualize the dependency better.

[github-actions]

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- set default `max_spans` to 1000 ([#1132](https://github.com/getsentry/sentry-native/pull/1132))

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