[Internal] Use gofumpt instead of gofmt

[mgyucht]

Changes

Gofumpt is a stricter version of gofmt. This is used in the cli repo, and we would like to adopt it in the TF provider as well.

In a follow-up PR, we will add golangci-lint using similar rules to the CLI repo: https://github.com/databricks/cli/blob/main/.golangci.yaml

Tests

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

[github-actions]

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: 4448
• Commit SHA: 6021530ab15391ee82803b62af66b3cb52318d62

Checks will be approved automatically on success.