feat: Add ovh_savings_plan resource

[amstuta]

Description

Add resource ovh_savings_plan.

Fixes #769

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (improve existing resource(s) or datasource(s))
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Test A: make testacc TESTARGS="-run TestAccResourceSavingsPlan_basic"

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings or issues
• I have added acceptance tests that prove my fix is effective or that my feature works
• New and existing acceptance tests pass locally with my changes
• I ran succesfully go mod vendor if I added or modify go.mod file

[deathiop]

Suggested change

	d.Set("auto_renewal", autoRenewal)
	d.Set("auto_renewal", resp.PeriodEndAction == "REACTIVATE")