feat: Add datasources for Rancher reference data

[amstuta]

Description

Add datasources

• ovh_cloud_project_rancher_version
• ovh_cloud_project_rancher_plan

Fixes #847 (issue)
Fixes #848 (issue)

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• Test A: make testacc TESTARGS="-run TestAccDataSourceCloudProjectRancherVersion_basic"
• Test B: make testacc TESTARGS="-run TestAccDataSourceCloudProjectRancherPlan_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