fix: Fix broken serialization of node transaction spans

[timfish]

Fixes #418

The Electron SDK has a mergeEvents function which uses deepmerge. This saves having a large manual merge function which has to be kept up to date if any of the event objects change.

The Sentry tracing Span has a custom toJSON implementation to convert camelCase properties to snake_case to match the Sentry API guidelines. When the event gets merged with the defaults this function is lost.

This PR:

• Manually copies the spans so their toJSON function is still there
• Adds an e2e test to ensure this doesn't break in the future
• Improves the e2e event normalisation code so it doesn't add properties

[AbhiPrasad]

Do we need to do similar for sessions as well? I assume it's not a problem for now because sessions don't got through the event processors or from renderer -> main process.

[timfish]

Do we need to do similar for sessions as well?

I think not. Sessions don't go through the event processors and we don't touch them otherwise.