fix(near-primitives): EpochReference enum can be flattened into a struct

[miraclx]

EpochReference is defined as such:

nearcore/core/primitives/src/types.rs

Lines 1001 to 1007 in 06d05bc

	#[derive(Serialize, Deserialize, Debug)]
	#[serde(rename_all = "snake_case")]
	pub enum EpochReference {
	EpochId(EpochId),
	BlockId(BlockId),
	Latest,
	}

And it's serialization, as such:

%	EpochReference::BlockId(...)	EpochReference::Latest
expectation	{"block_id": ...}	{"latest": null}
reality	{"block_id": ...}	"latest"

As you can already see, serde's default method for serializing unit variants is to use a string
unlike how it uses an object for the rest of the variants.

This PR implements custom serialization for EpochReference, specifically for the Latest variant. So it serializes to {"latest": null}.

Note: this retains the default deserialization so we have no breaking changes on the deserialization side.

So, the following values properly deserialize into that variant:

• "latest"
• {"latest": null}
• {"latest": []}

props to @volovyk-s 🎉 for discovering this while using the new near-jsonrpc-client

[miraclx]

More context:

In the case where it's embedded in a struct whose serialization requires flattening EpochReference, for example:

nearcore/chain/jsonrpc-primitives/src/types/validator.rs

Lines 18 to 22 in 06d05bc

	#[derive(Serialize, Deserialize, Debug)]
	pub struct RpcValidatorRequest {
	#[serde(flatten)]
	pub epoch_reference: near_primitives::types::EpochReference,
	}

All is well until you use the Latest variant of EpochReference.

code	expectation	reality
RpcValidatorRequest { epoch_reference: EpochReference::BlockId(...) };	{ "block_id": ... }
RpcValidatorRequest { epoch_reference: EpochReference::Latest };	{ "latest": null }	Error: can only flatten structs and maps (got an enum)

[frol]

Meh, this somehow slipped through my review, but the API is quite odd. I am fine with this hotfix, though I want to share more context here.

validators({block_id: 123})
validators({block_id: "deadbeef"})
validators({latest: null})

We have not even updated the documentation: https://docs.near.org/docs/api/rpc/network#validation-status, so we may as well refactor the API altogether.

validators({epoch_reference: {block_id: 123}})
validators({epoch_reference: {block_id: "deadbeef"}})
validators({epoch_reference: "latest"})

Maybe you have a better idea, I am currently not at my highest with naming 😞

[matklad]

It's not immediately obvious that derive serialization can deserialize our custom one, so it might make sense to add a unit-test for this.