feat: add workspace auto destroy duration field

[notchairmk]

Description

Adds an auto_destroy_activity_duration field to Workspaces (see API docs). Makes use of jsonapi.NullableAttr because this is an optional string field, but we will want to be able to send an explicit null to unset.

Testing plan

1. Run auto destroy tests, confirm that they're valid envchain go-tfe go test -run "TestWorkspacesAutoDestroy.*" -v ./...

External links

Output from tests

Including output from tests may require access to a TFE instance. Ignore this section if you have no environment to test against.

$ TFE_ADDRESS="https://example" TFE_TOKEN="example" go test ./... -v -run "TestWorkspacesAutoDestroy.*"

?   	github.com/hashicorp/go-tfe/examples/backing_data	[no test files]
?   	github.com/hashicorp/go-tfe/examples/configuration_versions	[no test files]
?   	github.com/hashicorp/go-tfe/examples/organizations	[no test files]
?   	github.com/hashicorp/go-tfe/examples/registry_modules	[no test files]
?   	github.com/hashicorp/go-tfe/examples/run_errors	[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   TestWorkspacesAutoDestroy
--- PASS: TestWorkspacesAutoDestroy (4.72s)
=== RUN   TestWorkspacesAutoDestroyDuration
--- PASS: TestWorkspacesAutoDestroyDuration (3.85s)
PASS
ok  	github.com/hashicorp/go-tfe	9.099s
...

[notchairmk]

swapped the order on these since require wants require.Equal(t, <expected>, <actual>)

[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.