spanconfigstore: pull out the span config storage into internal struct

This commit pulls out what used to be the spanconfig.Store logic into
as separate spanconfigstore.spanConfigStore struct to exclusively track
span <-> config pairs. This also makes room for adding another internal
struct to track system span configs in a future patch.

Release note: None

pkg/spanconfig/spanconfigstore/BUILD.bazel

@@ -2,7 +2,10 @@ load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
 
 go_library(
     name = "spanconfigstore",
-    srcs = ["store.go"],
+    srcs = [
+        "spanconfigstore.go",
+        "store.go",
+    ],
     importpath = "github.com/cockroachdb/cockroach/pkg/spanconfig/spanconfigstore",
     visibility = ["//visibility:public"],
     deps = [
@@ -20,7 +23,10 @@ go_library(
 
 go_test(
     name = "spanconfigstore_test",
-    srcs = ["store_test.go"],
+    srcs = [
+        "spanconfigstore_test.go",
+        "store_test.go",
+    ],
     data = glob(["testdata/**"]),
     embed = [":spanconfigstore"],
     deps = [