cloudflare_argo_tunnel: add tunnel token to state

[bradleyglover]

The cloudflared service supports passing in a token to allow a connector to authenticate and run a tunnel. This token is a base64 encoded string made up of the account ID, tunnel ID, and secret ID. Since these items are already stored in state, this PR represents a nice quality of life improvement by constructing, encoding, and storing the token in state too.

Closes #1439

[jacobbednarz]

i'm going to put this one on the back burner until we land the changes from cloudflare/cloudflare-go#866 and update the methods internally. as is, this won't work because the tunnel secret is only available on the Create response; not the individual fetch. however, we did recently add a method to directly get the token via https://github.com/cloudflare/cloudflare-go/blob/master/tunnel.go#L266 which we can use instead.

[jacobbednarz]

the upstream changes landed this morning if you'd like to incorporate them into this PR and we can get it merged.

[bradleyglover]

Nice, I'll take a stab at refactoring this tomorrow or the next day.

Looking at #1439 (comment), it seems we will expect errors setting the token state for older tunnels. Is there any example I can lean on for how the provider would handle that? aka leave the token field empty, return a message, etc?

[jacobbednarz]

for older tunnels that return the 404, let's leave it empty. we can issue a diag.Warning from the Read operation to notify the end user it won't be set and perhaps recreate it if they want the token to work in Terraform. since we have a way to fetch the accurate value, we can't populate it with dummy or wrong data as people may rely on it.

[bradleyglover]

Updated to use the upstream TunnelToken method, but this will not be functional until the changes from cloudflare/cloudflare-go#881 are released

[jacobbednarz]

@bradleyglover this is ready for you to pick back up and test now.

[bradleyglover]

@jacobbednarz thanks! all tests passed for me locally (with the exception of some existing lint failures)

[github-actions]

This functionality has been released in v3.16.0 of the Terraform Cloudflare Provider.

Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!