Add the ability to specify custom shellcraft templates

Shellcraft templates are a really nice feature when needing to write custom shellcode but there is currently no easy way to do this.
The best way I can come up with is something like the following:
```python
from pwn import shellcraft
from pwnlib.shellcraft.internal import make_function
import shutil

shellcraft_templates = Path(shellcraft.__file__).resolve().parent / "templates"
shutil.copy("custom_shellcode.asm", shellcraft_templates / "x86" / "linux")
custom_shellcode = make_function("custom_shellcode", "custom_shellcode.asm", "x86/linux")
...
```
But copying files to shellcraft's template folder and relying on internal interfaces to do this does not seem ideal

Start by reading pwnlib.shellcraft.internal.make_function and the shellcraft templates directory described in the issue, including the custom_shellcode.asm example. Define and verify a supported way to load a custom template without copying files into the installed template folder or relying on internal interfaces.

Written by the indexing model from the issue text.