Enable Terraform Cloud joining for Teleport Community Edition (#46419)

* Terraform Cloud joining: Support Terraform Enterprise issuers

This adds support for hostname/issuer overrides, needed to support
on-prem Terraform Enterprise installs. When the new `hostname` field
is unset, behavior is changed, but when set, the JWT is validated
against it instead of `app.terraform.io`.

Additionally, this renames `join_terraform.go` to
`join_terraformcloud.go`, since that was missed during the rename
in #45574.

* Enable Terraform Cloud joining for Teleport Community Edition

This enables Terraform Cloud joining for Community Edition when
using the public HCP Terraform SaaS. Teleport Enterprise is still
required for use with self-hosted Terraform Enterprise.

changelog: Enable Terraform Cloud joining for Teleport Community Edition when using HCP Terraform

* Fix unit tests

* Update lib/auth/join_terraformcloud.go

Co-authored-by: Tiago Silva <tiago.silva@goteleport.com>

* Fix linter

---------

Co-authored-by: Tiago Silva <tiago.silva@goteleport.com>

lib/auth/join_terraformcloud.go

@@ -50,9 +50,10 @@ func (a *Server) checkTerraformCloudJoinRequest(ctx context.Context, req *types.
 		return nil, trace.BadParameter("terraform_cloud join method only supports ProvisionTokenV2, '%T' was provided", pt)
 	}
 
-	if modules.GetModules().BuildType() != modules.BuildEnterprise {
+	hostnameOverride := token.Spec.TerraformCloud.Hostname
+	if hostnameOverride != "" && modules.GetModules().BuildType() != modules.BuildEnterprise {
 		return nil, fmt.Errorf(
-			"terraform_cloud joining: %w",
+			"terraform_cloud joining for Terraform Enterprise: %w",
 			ErrRequiresEnterprise,
 		)
 	}
@@ -68,7 +69,7 @@ func (a *Server) checkTerraformCloudJoinRequest(ctx context.Context, req *types.
 	}
 
 	claims, err := a.terraformIDTokenValidator.Validate(
-		ctx, aud, token.Spec.TerraformCloud.Hostname, req.IDToken,
+		ctx, aud, hostnameOverride, req.IDToken,
 	)
 	if err != nil {
 		return nil, trace.Wrap(err)

lib/auth/join_terraformcloud_test.go

@@ -155,6 +155,7 @@ func TestAuth_RegisterUsingToken_Terraform(t *testing.T) {
 							ProjectName:      "example-project",
 						},
 					},
+					Hostname: "terraform.example.com",
 				},
 			},
 			request: newRequest(validIDToken),