[FEATURE] Deep merge for environment inputs in bundle instances

[cwe1ss]

Is your feature request related to a problem? Please describe.
Our modules often include variables with complex object({...} types. E.g., for configuring the network of a project, some attributes of that network variable are environment-specific (e.g. the cidr_range for the VNet), and others are the same for all environments (e.g., a peering_enabled-flag, which subnets to create, or the delegations of a subnet).

variable "network" {
  type = object({
    peering_enabled = bool
    cidr_range      = string
    subnets         = map(object({
      cidr_range  = string
      delegations = optional(list(string), [])
    }))
  })
}

Describe the solution you'd like
Please use the same deep merge algorithm for resolving environment-inputs in bundle instances like you're using for resolving globals in a stack. This means, that objects/maps should be merged, and lists should be replaced.

This would allow us to configure a bundle instance like this:

apiVersion: terramate.io/cli/v1
kind: BundleInstance
spec:
  source: /bundles/subscription
  inputs:

    # Settings that are the same for all environments
    network:
      peering_enabled: true
      subnets:
        containers:
          delegations: ["Microsoft.App/environments"]

environments:
  dev:
    inputs:
      network:
        cidr_range: "192.168.1.0/24"
        subnets:
          containers:
            cidr_range: "192.168.1.0/24"
  prod:
    inputs:
      network:
        cidr_range: "192.168.2.0/24"
        subnets:
          containers:
            cidr_range: "192.168.2.0/24"

Describe alternatives you've considered

• Use flat inputs, but this doesn't work for maps and list of objects.
• Define every input for every environments.{env}.inputs and don't set the inputs on spec.inputs, but this leaves very little usefulness to the environment-feature. we might as well just use separate bundle instances for each environment.

Additional context
Here's an example from my playground repo. the prod-environment should use the value from internet_allowed_fqdns defined in spec.inputs.network, but it does not, since the entire network input is overwritten.

Atmos also uses deep merging when resolving all the yaml-files for a component.