[lib] add post cast to neynar client

Summary:
This implements posting a cast using the neynar client.

Followed https://docs.neynar.com/reference/post-cast almost exactly.

Depends on D13783

Test Plan:
Unfortunately, something seems to be wrong with the signer uuid configured in our comm neynar account. Debugging, but will ensure a reply cast is properly posted bofore landing

```
[NODEM]   code: 'NotFound',
[NODEM]   message: 'Signer not found.',
[NODEM]   property: 'signer_uuid'
[NODEM] }
```

Reviewers: ashoat, varun

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13784

lib/types/farcaster-types.js

@@ -84,3 +84,8 @@ export type NeynarWebhookCastCreatedEvent = {
   +data: NeynarWebhookCastCreatedData,
   ...
 };
+
+export type NeynarPostCastResponse = {
+  +success: boolean,
+  ...
+};

lib/utils/neynar-client.js

@@ -7,6 +7,7 @@ import type {
   NeynarChannel,
   NeynarUser,
   NeynarCast,
+  NeynarPostCastResponse,
 } from '../types/farcaster-types.js';
 
 type FetchRelevantFollowersResponse = {
@@ -357,6 +358,38 @@ class NeynarClient {
       throw error;
     }
   }
+
+  async postCast(
+    signerUUID: string,
+    parent: string,
+    text: string,
+  ): Promise<NeynarPostCastResponse> {
+    const url = getNeynarURL('2', 'cast', {});
+    const body = {
+      signer_uuid: signerUUID,
+      parent,
+      text,
+    };
+    try {
+      const response = await fetch(url, {
+        method: 'POST',
+        headers: {
+          'Accept': 'application/json',
+          'api_key': this.apiKey,
+          'Content-Type': 'application/json',
+        },
+        body: JSON.stringify(body),
+      });
+
+      return await response.json();
+    } catch (error) {
+      console.log(
+        'Failed to post Farcaster cast:',
+        getMessageForException(error) ?? 'unknown',
+      );
+      throw error;
+    }
+  }
 }
 
 export { NeynarClient };