Implement AssetMetadataByIds endpoint of ShieldedPool service

[turbocrime]

penumbra/crates/core/component/shielded-pool/src/component/rpc.rs

Lines 67 to 72 in 64f3399

	async fn asset_metadata_by_ids(
	&self,
	_request: tonic::Request<AssetMetadataByIdsRequest>,
	) -> Result<tonic::Response<Self::AssetMetadataByIdsStream>, tonic::Status> {
	unimplemented!("asset_metadata_by_ids not yet implemented")
	}

any client using the web block processor will be ratelimited immediately when attempting genesis block sync. this is due to the massive number of requests issued during initial validator discovery (currently, there are 437 validators).

PR exists at penumbra-zone/web#1392 in web to avoid the behavior, but most of these requests could become a single request if this batch request endpoint were implemented.

[Valentine1898]

This is a duplicate of #4162 that was closed in favor of penumbra-zone/web#1189