Stream attachments from file to use in transport

[supervacuus]

This is a follow-up issue to #1001.

We currently load attachments via sentry__envelope_add_from_path into memory before adding them to the internal envelope structure:

sentry-native/src/sentry_envelope.c

Lines 358 to 375 in 45fa26b

	sentry_envelope_item_t *
	sentry__envelope_add_from_path(
	sentry_envelope_t *envelope, const sentry_path_t *path, const char *type)
	{
	if (!envelope) {
	return NULL;
	}
	size_t buf_len;
	char *buf = sentry__path_read_to_buffer(path, &buf_len);
	if (!buf) {
	SENTRY_WARNF("failed to read envelope item from \"%" SENTRY_PATH_PRI
	"\"",
	path->path);
	return NULL;
	}
	// NOTE: function will free `buf` on error
	return envelope_add_from_owned_buffer(envelope, buf, buf_len, type);
	}

Since #1001 relates to a customer issue where the SDK sent incomplete envelopes due to OOM, we could further reduce the required memory by not loading attachments into memory when constructing the envelope and deferring until the transport