[lib] include embeds in neynar client's postCast method

Summary:
This adds embeds as a parameter to the postCast method for the neynar client and makes text optional

Depends on D14086

Test Plan: Tested later in the stack. Was able to successfully post the frame url to warpcast

Reviewers: ashoat, varun

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D14092