Add automatic @timestamp field generation for Elasticsearch documents

[Copilot]

Documents sent from Kafka to Elasticsearch lack timestamp fields, preventing time-based queries and visualizations.

Changes

• New TimestampFilter: Adds @timestamp field (ISO 8601 format) to documents lacking one
• Configuration: AddTimestamp boolean in ElasticSearchConf (defaults to true)
• Filter ordering: Timestamp filter runs first, ensuring timestamps exist before user filters execute
• Preservation: Existing @timestamp fields are left untouched

Usage

Output:
  ElasticSearch:
    Hosts:
      - "http://172.16.188.73:9200"
    Index: "go-stash-{{yyyy.MM.dd}}"
    AddTimestamp: true  # Disable with false if needed

Documents without @timestamp will now receive 2006-01-02T15:04:05.000Z formatted timestamps in UTC.

Original prompt

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This section details on the original issue you should resolve

<issue_title>go-stash支持自动生成时间戳吗</issue_title>
<issue_description>
我使用go-stash订阅kafka，然后吐给es，但是数据里没有时间戳</issue_description>

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

• Fixes go-stash支持自动生成时间戳吗 #15

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