[antlir systemd timer API] Stop hard coding TimeoutStartSec=15min a…

…nd make it configurable

Summary:
I realised, when I created this API, that I had hard coded this...
Let's make this configurable.

Test Plan:
I kept the public facing macro defaulting to 15m so this will be a NOOP as I manually tested:

```
# pallotron @ devvm3927 in ~/fbcode [0:26:16]
$ buck run '//netos/os:os[container]'
File changed: fbcode//antlir/bzl/linux/systemd/service_unit.jinja2
File changed: fbcode//antlir/bzl/linux/systemd
File changed: fbcode//antlir/bzl
3 additional file change events
Buck UI: https://www.internalfb.com/buck2/29417122-84a5-4da1-b482-c554323a5edb
Network: Up: 4.9MiB  Down: 5.7GiB  (reSessionID-927494fa-ef86-4736-9947-81106077633f)
Analyzing targets. Remaining     0/101                                                                                                           7 actions
Executing actions. Remaining     0/45605                                                                                                         56:13:47.2s exec time total
Command: run.      Finished 63 local, 168 remote, 42119 cache (99% hit)                                                                          55:48:28.4s exec time cached (99%)
Time elapsed: 4:15.1s
BUILD SUCCEEDED
bash-5.1#
```

```
bash-5.1# cat /usr/lib/systemd/system/fb_procprint.service  | grep TimeoutStart
TimeoutStartSec=15m
```

Reviewed By: lsiudut

Differential Revision: D68008374

fbshipit-source-id: 891db5b23a63ab47a687ef04c02e45c6ea7e8e0d

antlir/bzl/linux/systemd/service_unit.jinja2

@@ -12,7 +12,9 @@ EnvironmentFile={{ environment_file }}
 {% for cmd in exec_start %}
 ExecStart={{ cmd }}
 {% endfor %}
-TimeoutStartSec=15min
+{% if timeout_start %}
+TimeoutStartSec={{ timeout_start }}
+{% endif %}
 TimeoutStopSec=90s
 {% for key, val in environment.items() %}
 Environment={{ key }}={{ val }}

antlir/bzl/linux/systemd/systemd.shape.bzl

@@ -22,6 +22,7 @@ service_t = shape.shape(
     environment = shape.field(shape.dict(str, str), optional = True),
     exec_start = shape.field(shape.list(str), optional = True),
     standard_output = shape.field(str, optional = True),
+    timeout_start = shape.field(str, optional = True),
 )
 
 timer_t = shape.shape(