Placeholders such as ${cert_subject} in JWT AuthN do not work about emqx HOT 4 OPEN

@zmstone But EMQX documentation does not mention that the placeholders supported by JWT authn are different from other authn: authentication-placeholders

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After this issue is fixed, there is still a risk of crash.

In function replace_placeholder of module emqx_authn_jwt.erl:

replace_placeholder(L, Variables) ->
    replace_placeholder(L, Variables, []).

replace_placeholder([], _Variables, Acc) ->
    Acc;
replace_placeholder([{Name, {placeholder, PL}} | More], Variables, Acc) ->
    Value = maps:get(PL, Variables),
    replace_placeholder(More, Variables, [{Name, Value} | Acc]);
replace_placeholder([{Name, Value} | More], Variables, Acc) ->
    replace_placeholder(More, Variables, [{Name, Value} | Acc]).

If there is no corresponding key in Variables, then maps:get(PL, Variables) will crash, as mentioned in Issue#13253, this is possible when fail_if_no_peer_cert is set to false.

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This is not a bug, but a feature request.
JWT authn only supports placeholders for clientid and username to check against the JWT claims.
A workaround for now is to use peer_cert_as_username or peer_cert_as_clientid.

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I found it: JWT AuthN

Maybe it's time to support it.

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