fix: add image option to signUpEmail types and schema (#3357)

`signUpEmail` destructures the optional `image` field from the request body and uses it when creating the user, but the field isn’t in the `requestBody` schema or the `$Infer` `body` type

packages/better-auth/src/api/routes/sign-up.test.ts

@@ -40,6 +40,7 @@ describe("sign-up with custom fields", async (it) => {
 				email: "email@test.com",
 				password: "password",
 				name: "Test Name",
+				image: "https://picsum.photos/200",
 			},
 		});
 		expect(res.token).toBeDefined();

packages/better-auth/src/api/routes/sign-up.ts

@@ -24,6 +24,7 @@ export const signUpEmail = <O extends BetterAuthOptions>() =>
 						name: string;
 						email: string;
 						password: string;
+						image?: string;
 						callbackURL?: string;
 						rememberMe?: boolean;
 					} & AdditionalUserFieldsInput<O>,
@@ -48,6 +49,10 @@ export const signUpEmail = <O extends BetterAuthOptions>() =>
 											type: "string",
 											description: "The password of the user",
 										},
+										image: {
+											type: "string",
+											description: "The profile image URL of the user",
+										},
 										callbackURL: {
 											type: "string",
 											description: