chore: replace manual value extraction

[ghost]

The TODO can be removed as the API discussed here was implemented and in a separate PR the json API for Value was used.

Before:

near-sdk-rs/examples/fungible-token/tests/workspaces.rs

Lines 220 to 229 in 7d31a5b

	// TODO: replace the following manual value extraction when workspaces
	// resolves https://github.com/near/workspaces-rs/issues/201
	match res.receipt_outcomes()[5].clone().into_result()? {
	ValueOrReceiptId::Value(val) => {
	let bytes = base64::decode(&val)?;
	let used_amount = serde_json::from_slice::<U128>(&bytes)?;
	assert_eq!(used_amount, transfer_amount);
	}
	_ => panic!("Unexpected receipt id"),
	}

After:

near-sdk-rs/examples/fungible-token/tests/workspaces.rs

Lines 202 to 210 in 6c8556e

	// TODO: replace the following manual value extraction when workspaces
	// resolves https://github.com/near/workspaces-rs/issues/201
	match res.receipt_outcomes()[5].clone().into_result()? {
	ValueOrReceiptId::Value(val) => {
	let used_amount = val.json::<U128>()?;
	assert_eq!(used_amount, transfer_amount);
	}
	_ => panic!("Unexpected receipt id"),
	}

[uint]

Good catch!