This commit updates the API to pass the MemoryMaxMB field, and the CLI to show
the max set for the task.

Also, start parsing the MemoryMaxMB in hcl2, as it's set by tags.

A sample CLI output; note the additional `Max: ` for "task":

```
$ nomad alloc status 96fbeb0b
ID                  = 96fbeb0b-a0b3-aa95-62bf-b8a39492fd5c
[...]

Task "cgroup-fetcher" is "running"
Task Resources
CPU        Memory         Disk     Addresses
0/500 MHz  32 MiB/20 MiB  300 MiB

Task Events:
[...]

Task "task" is "running"
Task Resources
CPU        Memory          Disk     Addresses
0/500 MHz  176 KiB/20 MiB  300 MiB
           Max: 30 MiB

Task Events:
[...]
```