feat: add feature flag and test for force-unlock

[tobikris]

Our implementation is currently not able to be used with the force-unlock command of terraform. The reason is that terraform sends n empty body for the unlock request and we expect the lock info.

I added a testcase for this and improved the handling slightly - however there is no clear path to allowing the force-unlock. The lock ID is not even sent by the client - should we just unlock any lock? Unfortunately we cannot send an error message to the client either - so they just see something like

Failed to unlock state: Unexpected HTTP response code 400

This basically means that we currently do not have any possibility to unlock the state besides manipulating the lock backend directly.
What do you think?

See https://github.com/hashicorp/terraform/blob/main/internal/backend/remote-state/http/client.go#L125-L142 for reference.

[lu1as]

I guess force-unlock is a important feature and manipulating the lock in the backend is annoying (speaking from experience). My suggestion would be to introduce a feature flag for the native force-unlock command which just ignores the lock ID. Further, I would like to a separate API endpoint for offering a force-unlock with lock ID check. Fixing it upstream (probably in OpenTofu) seems to be no option, as it would break all existing HTTP backend implementations, right?

[lu1as]

@tobikris Do you agree with my approach?