feat: date-based Elasticsearch index names via strftime (#27)

[y-ken]

Summary

Implements #27: support Logstash-style date-based index names (e.g. myindex-20180831).

How it works

mysql_replicator_elasticsearch resolves the target index from the tag's
index_name segment. This PR expands strftime tokens in that segment using
the record's event time:

def resolve_index_name(index_name, time)
  return index_name unless index_name && index_name.include?('%')
  Time.at(time.to_i).strftime(index_name)
rescue => e
  log.warn "...failed to expand index name '#{index_name}': #{e.message}"
  index_name
end

Users opt in by putting the tokens in the index-name part of the input plugin's tag:

tag myindex-%Y%m%d.mytype.${event}.${primary_key}

Index names without a % are returned unchanged, so existing configurations
are unaffected (fully backward compatible).

Notes

• Uses the record's event time (deterministic, matches when the row was emitted).
• README documents the feature and the delete-vs-rotation caveat (date-rotated
  indices suit insert-only data, since a delete targets the index for the
  delete's own time).

Tests

• test_expands_strftime_tokens_in_index_name — a %Y%m%d tag produces a dated index.
• test_index_name_without_token_is_unchanged — pass-through for plain names.

Verified locally on Ruby 3.4 (18 tests, 0 failures).

Closes #27

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