rpc-v2: Enable the archive class of methods (#3017)

The
[archive](https://github.com/paritytech/json-rpc-interface-spec/blob/main/src/api/archive.md)
API is unstable and subject to change.

This PR enables the `archive` class of the RPC-V2 spec to substrate
based chains.

The `archive` API is enabled for archive nodes: 
- the state of the blocks is in archive mode
- the block's bodies are in archive mode

While at it, this PR extends the `BlocksPrunning` enum with an
`is_archive` helper to check if the pruning mode keeps the block's
bodies for long enough.

Defaults used for the `archive` API:
- a maximum of 5 responses are provided for descendants queries (this is
similar to chainHead)
- a maximum of 8 item queries are accepted at a time

Before stabilizing the API we should look into these defaults and adjust
after collecting some data.

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

substrate/client/service/src/builder.rs

@@ -63,7 +63,9 @@ use sc_rpc::{
 	system::SystemApiServer,
 	DenyUnsafe, SubscriptionTaskExecutor,
 };
-use sc_rpc_spec_v2::{chain_head::ChainHeadApiServer, transaction::TransactionApiServer};
+use sc_rpc_spec_v2::{
+	archive::ArchiveApiServer, chain_head::ChainHeadApiServer, transaction::TransactionApiServer,
+};
 use sc_telemetry::{telemetry, ConnectionMessage, Telemetry, TelemetryHandle, SUBSTRATE_INFO};
 use sc_transaction_pool_api::{MaintainedTransactionPool, TransactionPool};
 use sc_utils::mpsc::{tracing_unbounded, TracingUnboundedSender};
@@ -664,6 +666,26 @@ where
 	)
 	.into_rpc();
 
+	// Part of the RPC v2 spec.
+	// An archive node that can respond to the `archive` RPC-v2 queries is a node with:
+	// - state pruning in archive mode: The storage of blocks is kept around
+	// - block pruning in archive mode: The block's body is kept around
+	let is_archive_node = config.state_pruning.as_ref().map(|sp| sp.is_archive()).unwrap_or(false) &&
+		config.blocks_pruning.is_archive();
+	if is_archive_node {
+		let genesis_hash =
+			client.hash(Zero::zero()).ok().flatten().expect("Genesis block exists; qed");
+		let archive_v2 = sc_rpc_spec_v2::archive::Archive::new(
+			client.clone(),
+			backend.clone(),
+			genesis_hash,
+			// Defaults to sensible limits for the `Archive`.
+			sc_rpc_spec_v2::archive::ArchiveConfig::default(),
+		)
+		.into_rpc();
+		rpc_api.merge(archive_v2).map_err(|e| Error::Application(e.into()))?;
+	}
+
 	let author = sc_rpc::author::Author::new(
 		client.clone(),
 		transaction_pool,