fix: planetscale_branch data source crash

[joemiller]

Given a main.tf such as:

provider "planetscale" {}

resource "planetscale_database" "mydb" {
  name         = "mydb"
  organization = "planetscale-terraform-testing"

  allow_data_branching          = false
  cluster_size                  = "PS_10"
  default_branch                = "main"
  production_branch_web_console = false
  region                        = "us-east"
}

resource "planetscale_branch" "staging" {
  name          = "staging"
  database      = planetscale_database.mydb.name
  organization  = planetscale_database.mydb.organization
  parent_branch = planetscale_database.mydb.default_branch
  production    = false
}

data "planetscale_branch" "staging" {
  database     = planetscale_database.mydb.name
  organization = planetscale_database.mydb.organization
  name         = planetscale_branch.staging.name
}

output "branch" {
  value = data.planetscale_branch.staging
}

A terraform plan will crash with:

Planning failed. Terraform encountered an error while generating this plan.

╷
│ Error: Value Conversion Error
│
│ with data.planetscale_branch.staging,
│ An unexpected error was encountered trying to convert tftypes.Value into provider.branchDataSourceModel. This is always an error in the provider. Please report the following to the provider developer:
│
│ mismatch between struct and object: Object defines fields not found in struct: initial_restore_id.

Fix by adding the missing initial_restore_id field to the branchDataSourceModel struct and include a acceptance test to cover the planetscale_branch data source.