feat: add sentry_update_context

[JoshuaMoelans]

Adding this for downstream SDKs, so they can avoid fully overwriting pre-set context from our native SDK init.

Popped up as part of https://github.com/getsentry/sentry-xbox/pull/151 where the unreal integration test calls AddDeviceContext which does a plain set_by_key, making us lose device.X context set by the console SDK.

Now with sentry_update_context we only overwrite existing keys' values, and also add the new keys from the passed-in object to the context. For example

ctx={a:0, b:1, c:2}
sentry_update_context(ctx, {a:-1, d:3})
// ctx becomes {a:-1, b:1, c:2, d:3}

[github-actions]

	Messages
📖	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

Generated by 🚫 dangerJS against 7ddd241

[jpnurmi]

LGTM 👍