[Fix] Fix drift in `databricks_cluster` when `is_single_node` is set to `true`

[ashenm]

Changes

Fixes #4360

Tests

• [x] make test run locally
• [ ] relevant change in docs/ folder
• [ ] covered with integration tests in internal/acceptance
• [ ] using Go SDK
• [ ] using TF Plugin Framework

In addition to the unit tests, deployed provider locally with following resource definition and verified diff

resource "databricks_cluster" "single_node" {
  spark_version      = "15.4.x-scala2.12"
  single_user_name   = "[email protected]"
  runtime_engine     = "STANDARD"
  node_type_id       = "r5.large"
  kind               = "CLASSIC_PREVIEW"
  is_single_node     = true
  data_security_mode = "SINGLE_USER"
  cluster_name       = upper("demo-single-node")

  custom_tags = {
    "owner" = "ashenm"
  }
  autotermination_minutes = 10
}

[ashenm]

@alexott please take a stab at it when u get a slot

[alexott]

Please add tests for it - we have example of test for CustomizeDiff in resource_sql_table

[alexott]

@mgyucht given the work on porting clusters to the plugin framework - do we need this PR?

[mgyucht]

That work may take a bit longer than we thought to finish, so I'm OK merging incremental improvements like this.

[ashenm]

@alexott @mgyucht whenever y'all have a slot please take a stab

[alexott]

There is still a drift in spark_conf

[github-actions[bot]]

If integration tests don't run automatically, an authorized user can run them manually by following the instructions below:

Trigger: go/deco-tests-run/terraform

Inputs:

• PR number: 4416
• Commit SHA: 5e148173002b1734a528d05f8da889ee7ac23049

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