Add custom field definitions to channel

[caitlynstocker]

Pending advice from Modern Deployments. Please hold off on reviewing for now 🛑

Background 🌇

Custom field definitions were added to Channels as part of the Ephemeral Environments project, as a way to require custom fields on a release made into an ephemeral environment channel. The values for the custom fields on a release can then be used to name (or set other values within?) the ephemeral environments created from the release.

What's this? 🌵

We want to enable terraform users to set custom field definitions on channels in their terraform configuration.

What it should be doing so far is:
〰️ adding CustomFieldDefinitions to the Channel schema
〰️ adding expander and flattener functions for CustomFieldDefinitions
〰️ referencing these inside the Channel expander and flattener functions.

How to review 🔍

🚩 Carefully! This is my first time touching terraform or the go client. I've checked the changes carefully but it is very much a claude special.
🧪 You can test this by creating a channel in terraform and adding, editing and removing custom field definitions. Example terraform config:

resource "octopusdeploy_channel" "example" {
  name       = "Development Channel (OK to Delete)"
  project_id = "Projects-1"

  custom_field_definitions = [
    {
      field_name  = "Ticket Number"
      description = "The issue tracker ticket number associated with this deployment"
    },
    {
      field_name  = "Ticket Number 2"
      description = "Hello"
    }
  ]
}

Completes DEVEX-147

[caitlynstocker]

Autogenerated docs changes.

[caitlynstocker]

This needs to be updated to point to the latest release once changes to the go client are completed.

[caitlynstocker]

This will be removed once we are using the new release of the go client.

[caitlynstocker]

Replaced by #200