Add provisional for ConfigurationVersions; Add current-configuration-version to Workspaces

[brandonc]

Description

• Adds BETA field Provisional (does not set configuration version as workspace current)
• Adds CurrentConfigurationVersion relation to Workspace

Testing plan

See automated test output

$ ENABLE_BETA=1 go test ./... -run TestConfigurationVersionsCreate -v
?   	github.com/hashicorp/go-tfe/examples/configuration_versions	[no test files]
?   	github.com/hashicorp/go-tfe/examples/registry_modules	[no test files]
?   	github.com/hashicorp/go-tfe/examples/organizations	[no test files]
?   	github.com/hashicorp/go-tfe/examples/state_versions	[no test files]
?   	github.com/hashicorp/go-tfe/examples/users	[no test files]
?   	github.com/hashicorp/go-tfe/examples/workspaces	[no test files]
?   	github.com/hashicorp/go-tfe/mocks	[no test files]
=== RUN   TestConfigurationVersionsCreate
=== RUN   TestConfigurationVersionsCreate/with_valid_options
=== RUN   TestConfigurationVersionsCreate/with_invalid_workspace_id
=== RUN   TestConfigurationVersionsCreate/provisional
--- PASS: TestConfigurationVersionsCreate (16.65s)
    --- PASS: TestConfigurationVersionsCreate/with_valid_options (3.39s)
    --- PASS: TestConfigurationVersionsCreate/with_invalid_workspace_id (0.00s)
    --- PASS: TestConfigurationVersionsCreate/provisional (3.26s)
PASS
ok  	github.com/hashicorp/go-tfe	16.952s
...

[laurenolivia]

I added the DO NOT MERGE label so the hero after me does not merge this one bc the atlas PR hasn't merged. @brandonc

[Uk1288]

🎉 LGTM!

• todo: Update changelog to reflect the provisional attribute

[brandonc]

Merging without approval because the platform change is deployed and I updated the CHANGELOG

[github-actions]

Reminder to the contributor that merged this PR: if your changes have added important functionality or fixed a relevant bug, open a follow-up PR to update CHANGELOG.md with a note on your changes.