mark all tokens as sensitive

[matifali]

We should ensure that the following attributes are marked as sensitive.

I have seen an example in hcp-terraform-provider where they return data.hcp_vault_secrets_secret.example.secret_value as a sensitive value.

A related code snippet for this is:

https://github.com/hashicorp/terraform-provider-hcp/blob/d58d335805f6c67ecd26d547afdc10273aa96709/internal/provider/vaultsecrets/data_source_vault_secrets_secret.go#L54-L58

We are already doing it for

1. data.coder_workspace_owner.me.ssh_private_key
2. coder_agent.example.token

We should also do it for

1. data.coder_workspace_owner.me.oidc_access_token
2. data.coder_workspace_owner.me.session_token
3. data.coder_external_auth.example.access_token