The babel package supports a Bengali version of \arabic and (as an undocumented bonus feature) \alph. This enables you to format your example with enumitem, and also localizes the numbers of pages, chapters, sections, table of contents entries, etc.
\tracinglostchars=2 % Warn if a glyph is not in the current font
\documentclass[11pt]{article}
\usepackage{iftex}
\usepackage{enumitem}
\ifLuaTeX
\usepackage[english, bidi=basic, layout=sectioning.counters]{babel}
\usepackage{fontspec}
\defaultfontfeatures{Renderer=HarfBuzz, Scale=MatchLowercase, Ligatures = TeX}
\else % XeLaTeX?
\usepackage[english, bidi=default, layout=sectioning.counters]{babel}
\usepackage{fontspec}
\defaultfontfeatures{Scale=MatchLowercase, Ligatures = TeX}
\fi
\babelprovide[import, main, maparabic, alph=alphabetic]{bengali}
%% From: https://github.com/potasiyam/Kalpurush
%% A matching font family might be Gentium.
\babelfont{rm}
[Scale = 1.0,
BoldFont = *,
BoldFeatures = {FakeBold = 1.1}
]{Kalpurush}
\begin{document}
\section*{বাংলায় enumeration }
\begin{enumerate}[label=\arabic*।]
\item 1st item
\item 2nd item
\begin{enumerate}[label=(\alph*)]
\item 1st sub item
\item 2nd sub item
\end{enumerate}
\end{enumerate}
\end{document}

This requires LuaLaTeX 1.12.0 or greater, and as of June 2020, a bug in fontspec prevents this MWE from working properly in XeLaTeX. Until this is fixed, LuaLaTeX from TeX Live 2020 works, and XeLaTeX from TeX Live 2019 might too.
If these don’t meet your needs, you will need to define your own enumeration counter.