[CAL-4974] Add custom payload Input format

[Shu12388y]

When we create a webhook there are two options first one is default and the second one is the custom payload in the custom payload there is no structure defining how to send and what we can send in the payload

Describe the solution you'd like

We can define the custom payload and add we can also add a placeholder inside the custom payload so that the user cannot get confused

@Praashh can you assign this issue to me

_CAL-4974

[Praashh]

we don't assign issues, feel free to create a Pr

[Shu12388y]

ok @Praashh