rpc: unconstrain meta field for MintAssets

In this commit, we stop trying to strictly parse the meta field.
Instead, users can set it to w/e they want, with the default being
opaque. This lets users start to add structure to the meta field from
day one. One example is a set meta with fields that dictate how the
asset unit is to be displayed. This can be used to do things like
leverage the current fix point format to add decimal places for display.

rpcserver.go

@@ -7,6 +7,7 @@ import (
 	"encoding/hex"
 	"errors"
 	"fmt"
+	"math"
 	"net/http"
 	"strconv"
 	"strings"
@@ -372,13 +373,15 @@ func (r *rpcServer) MintAsset(ctx context.Context,
 	}
 
 	if req.Asset.AssetMeta != nil {
-		metaType, err := unmarshalMetaType(req.Asset.AssetMeta.Type)
-		if err != nil {
-			return nil, err
+		// Ensure that the meta field is within bounds.
+		if req.Asset.AssetMeta.Type > math.MaxUint8 {
+			return nil, fmt.Errorf("meta type is too large: %v, "+
+				"max is: %v", req.Asset.AssetMeta.Type,
+				math.MaxUint8)
 		}
 
 		seedling.Meta = &proof.MetaReveal{
-			Type: metaType,
+			Type: proof.MetaType(req.Asset.AssetMeta.Type),
 			Data: req.Asset.AssetMeta.Data,
 		}
 
@@ -3960,17 +3963,6 @@ func (r *rpcServer) RemoveUTXOLease(ctx context.Context,
 	return &wrpc.RemoveUTXOLeaseResponse{}, nil
 }
 
-// unmarshalMetaType maps an RPC meta type into a concrete type.
-func unmarshalMetaType(rpcMeta taprpc.AssetMetaType) (proof.MetaType, error) {
-	switch rpcMeta {
-	case taprpc.AssetMetaType_META_TYPE_OPAQUE:
-		return proof.MetaOpaque, nil
-
-	default:
-		return 0, fmt.Errorf("unknown meta type: %v", rpcMeta)
-	}
-}
-
 // MarshalAssetFedSyncCfg returns an RPC ready asset specific federation sync
 // config.
 func MarshalAssetFedSyncCfg(