feat: plugin-managed Elasticsearch index templates (#20, #36 geo_point)

[y-ken]

Summary

Lets mysql_replicator_elasticsearch install an index template on startup so newly created indices get the desired mapping
before the first document locks in dynamic mapping.

This unblocks the mapping-dependent requests that value coercion can't reach:

• keyword / not_analyzed fields ( How to set “index”: “not_analyzed” While push data from Mysql using mysql-replicator  #20)
• geo_point (mysql and es datatype support  #36) — dynamic mapping never infers it

Config (mirrors fluent-plugin-elasticsearch)

template_name        myindex_template
template_file        /etc/fluent/myindex_template.json
template_overwrite   false
use_legacy_template  true   # true: PUT /_template (ES 6.x+); false: PUT /_index_template (ES >= 7.8)

template_name + template_file must be set together. Option names, defaults,
and template-file formats match fluent-plugin-elasticsearch, so existing
templates/knowledge carry over.

Why this is the right mechanism for date-based indices

The template is a server-side rule: once installed, Elasticsearch applies it
to every new index matching index_patterns, including the next day's
date-rolled index (from #54) — no per-write or per-day action needed. This is
exactly why a template (not per-index creation) is used.

Behavior / safety

• Installed once on the first write (after version detection); failures are
  logged but never abort indexing.
• Composable templates (use_legacy_template false) require ES ≥ 7.8; on older
  ES it is skipped with a warning. The default legacy path works on ES 6.x+, so
  this is an opt-in, backward-compatible addition (no change to core
  indexing or the supported ES range) → next release is a minor (v1.4.0).

Example template_file (legacy format, default)

{
  "index_patterns": ["myindex-*"],
  "mappings": { "properties": {
    "message":  { "type": "keyword" },
    "location": { "type": "geo_point" }
  } }
}

Tests

5 new unit tests (WebMock): legacy install (default), composable install
(use_legacy_template false + ES 8.x), skip on ES < 7.8, skip when the template
exists and template_overwrite false, and the must-be-set-together config error.
Verified locally on Ruby 3.4 (24 tests, 0 failures).

Follow-ups

• After merge: comment on / close How to set “index”: “not_analyzed” While push data from Mysql using mysql-replicator  #20 (keyword via template now documented); on
  mysql and es datatype support  #36, the geo_point half is covered here, the decimal→numeric half remains
  (value coercion).

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