feat(shared): implement slingshot identity resolver

packages/shared/deno.jsonc

···
+{
+  "name": "@cistern/shared",
+  "exports": {
+    ".": "./mod.ts"
+  },
+  "imports": {
+    "@atcute/lexicons": "npm:@atcute/lexicons@^1.2.2"
+  }
+}

packages/shared/mod.ts

···
+export * from "./resolve-did.ts";

packages/shared/resolve-did.ts

···
+import type { Did, Handle } from "@atcute/lexicons";
+
+export interface MiniDoc {
+  did: Did;
+  handle: Handle;
+  pds: string;
+  signing_key: string;
+}
+
+export async function resolveMiniDoc(
+  handle: string,
+  slingshotUrl?: string,
+): Promise<MiniDoc> {
+  const url = new URL(
+    "/xrpc/com.bad-example.identity.resolveMiniDoc",
+    slingshotUrl ?? "https://slingshot.microcosm.blue",
+  );
+
+  url.searchParams.set("handle", handle);
+
+  const result = await fetch(url.toString());
+
+  if (!result.ok) {
+    throw new Error(
+      `failed to fetch identity mini doc: Error ${result.status} ${result.statusText}`,
+    );
+  }
+
+  return await result.json();
+}