chore: per template opt into cached terraform directories (#20609)

For experimental and dogfood purposes, this adds the ability to opt in a single template. 
Leaving the rest of the templates as is. 

For GA, this setting might be removed or changed.

coderd/apidoc/swagger.json

@@ -16613,6 +16613,9 @@
 				},
 				"use_classic_parameter_flow": {
 					"type": "boolean"
+				},
+				"use_terraform_workspace_cache": {
+					"type": "boolean"
 				}
 			}
 		},
@@ -17497,6 +17500,10 @@
 				"use_classic_parameter_flow": {
 					"description": "UseClassicParameterFlow is a flag that switches the default behavior to use the classic\nparameter flow when creating a workspace. This only affects deployments with the experiment\n\"dynamic-parameters\" enabled. This setting will live for a period after the experiment is\nmade the default.\nAn \"opt-out\" is present in case the new feature breaks some existing templates.",
 					"type": "boolean"
+				},
+				"use_terraform_workspace_cache": {
+					"description": "UseTerraformWorkspaceCache allows optionally specifying whether to use cached\nterraform directories for workspaces created from this template. This field\nonly applies when the correct experiment is enabled. This field is subject to\nbeing removed in the future.",
+					"type": "boolean"
 				}
 			}
 		},