[#705] pkg/innerring: Add IR keys fetchers

Add `IrFetcherWithNotary` and `IrFetcherWithoutNotary`
that can fetch IR keys with and without usage of
notary contract.
Both can be hidden behind
`InnerRingKeys() (keys.PublicKeys, error)` interface.

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

pkg/innerring/fetcher.go

@@ -0,0 +1,58 @@
+package innerring
+
+import (
+	"github.com/nspcc-dev/neo-go/pkg/crypto/keys"
+	"github.com/nspcc-dev/neofs-node/pkg/morph/client"
+	nmWrapper "github.com/nspcc-dev/neofs-node/pkg/morph/client/netmap/wrapper"
+)
+
+// NewIRFetcherWithNotary creates IrFetcherWithNotary.
+//
+// IrFetcherWithNotary can be used to obtain innerring key list if
+// network that client is connected to supports notary contract.
+//
+// Passed client is required. Panics if nil.
+func NewIRFetcherWithNotary(cli *client.Client) *IrFetcherWithNotary {
+	if cli == nil {
+		panic("could not init IRFetcher with notary: client must not be nil")
+	}
+	return &IrFetcherWithNotary{cli: cli}
+}
+
+// NewIRFetcherWithoutNotary creates IrFetcherWithoutNotary.
+//
+// IrFetcherWithoutNotary must be used to obtain innerring key list if
+// network that netmap wrapper is connected to does not support notary
+// contract.
+//
+// Passed netmap wrapper is required. Panics if nil.
+func NewIRFetcherWithoutNotary(nm *nmWrapper.Wrapper) *IrFetcherWithoutNotary {
+	if nm == nil {
+		panic("could not init IRFetcher without notary: netmap wrapper must not be nil")
+	}
+	return &IrFetcherWithoutNotary{nm: nm}
+}
+
+// IrFetcherWithNotary fetches keys using notary contract. Must be created
+// with NewIRFetcherWithNotary.
+type IrFetcherWithNotary struct {
+	cli *client.Client
+}
+
+// IrFetcherWithoutNotary fetches keys using netmap contract. Must be created
+// with NewIRFetcherWithoutNotary.
+type IrFetcherWithoutNotary struct {
+	nm *nmWrapper.Wrapper
+}
+
+// InnerRingKeys fetches list of innerring keys from NeoFSAlphabet
+// role in side chain.
+func (fN IrFetcherWithNotary) InnerRingKeys() (keys.PublicKeys, error) {
+	return fN.cli.NeoFSAlphabetList()
+}
+
+// InnerRingKeys fetches list of innerring keys from netmap contract
+// in side chain.
+func (f IrFetcherWithoutNotary) InnerRingKeys() (keys.PublicKeys, error) {
+	return f.nm.GetInnerRingList()
+}