Add genitive (possessive form) for acro

```latex
\usepackage[single]{acro}
\acsetup{%
  group-citation = true,
  group-cite-cmd = {\noparencite},
}
\ProvideAcroEnding{possessive}{'s}{'s}
\ExplSyntaxOn
\NewAcroCommand \acg
{
  \acro_possessive:
  \acro_use:n {#1}
}
\NewAcroCommand \acsg
{
  \acro_possessive:
  \acro_short:n {#1}
}
\NewAcroCommand \aclg
{
  \acro_possessive:
  \acro_long:n {#1}
}
\ExplSyntaxOff
```