Attachments and SentryEnvelopeItemHeader should support the 'attachmentType' field

[marandaneto]

Right now attachments and envelope item headers only use the default value inferred by the backend, https://develop.sentry.dev/sdk/envelopes/#attachment which is event.attachment, but if an envelope is written by 3rd party SDKs like Flutter/RN/Sentry-native that uses a different type, it does not work.

Eg #1256

We'd need to support this field to not be lost during serialization/deserialization.

cc @philipphofmann

[marandaneto]

Done by #1504