Add IExchangeResultDataConvertor interface for customizable exchange result format

[Copilot]

Users had no way to customize the format of exchange rate data appended to API responses — it was hardcoded via ExchangeResultDto.toMap() returning {base, trans, rate, price}.

Changes

• New IExchangeResultDataConvertor interface (core/convertor/) — single convert(ExchangeResultDto) method returning Map<String, Object>
• DefaultExchangeResultDataConvertor — default implementation delegating to toMap(), preserving backward compatibility
• AutoApplyExchangeStrategy — now accepts IExchangeResultDataConvertor via constructor; replaces exchangeResult.toMap() with convertor.convert(exchangeResult)
• AutoExchangeAutoConfiguration — registers DefaultExchangeResultDataConvertor as @ConditionalOnMissingBean, allowing users to override with a custom @Bean

Usage

Override the default format by providing a custom bean:

@Bean
public IExchangeResultDataConvertor customConvertor() {
    return result -> {
        Map<String, Object> map = new HashMap<>();
        map.put("convertedPrice", result.getPrice());
        map.put("targetCurrency", result.getTransCurrency());
        return map;
    };
}

This replaces the default {base, trans, rate, price} structure with whatever shape the user defines.

Original prompt

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This section details on the original issue you should resolve

<issue_title>【新增】增加汇率转换信息的Convert方法，允许自定义追加格式</issue_title>
<issue_description>新增接口IExchangeResultDataConvertor接口，允许用户自定义追加的信息</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes 【新增】增加汇率转换信息的Convert方法，允许自定义追加格式 #2

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[juwencheng]

@cursor review 这个 PR，看看还有什么问题没有

[cursor]

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