drivers/exec+java: Add task configuration to restore previous PID/IPC isolation behavior

This PR adds pid_mode and ipc_mode options to the exec and java task
driver config options. By default these will defer to the default_pid_mode
and default_ipc_mode agent plugin options created in #9969. Setting
these values to "host" mode disables isolation for the task. Doing so
is not recommended, but may be necessary to support legacy job configurations.

Closes #9970

website/content/docs/drivers/java.mdx

@@ -48,6 +48,20 @@ The `java` driver supports the following configuration in the job spec:
 - `jvm_options` - (Optional) A list of JVM options to be passed while invoking
   java. These options are passed without being validated in any way by Nomad.
 
+- `pid_mode` - (Optional) Set to `"private"` to enable PID namespace isolation for
+  this task, or `"host"` to disable isolation. If left unset, the behavior is
+  determined from the [`default_pid_mode`][default_pid_mode] in plugin configuration.
+
+!> **Warning:** If set to `"host"`, other processes running as the same user will
+ be able to access sensitive process information like environment variables.
+
+- `ipc_mode` - (Optional) Set to `"private"` to enable IPC namespace isolation for
+  this task, or `"host"` to disable isolation. If left unset, the behavior is
+  determined from the [`default_ipc_mode`][default_ipc_mode] in plugin configuration.
+
+!> **Warning:** If set to `"host"`, other processes running as the same user will be
+ able to make use of IPC features, like sending unexpected POSIX signals.
+
 ## Examples
 
 A simple config block to run a Java Jar:
@@ -192,3 +206,6 @@ create.
 
 This list is configurable through the agent client
 [configuration file](/docs/configuration/client#chroot_env).
+
+[default_pid_mode]: /docs/drivers/java#default_pid_mode
+[default_ipc_mode]: /docs/drivers/java#default_ipc_mode