filter_lua: expose env variables in FLB_ENV Lua table

[edsiper]

This PR implements a new global Lua Table that register the Fluent Bit Environment variables.

Fixes #9585

example:

env:
  A: aaa
  B: bbb
  C: ccc

service:
    flush:           1
    log_level:       info

pipeline:
    inputs:
        - name:    random
          tag:     test
          samples: 10

    filters:
        - name:  lua
          match: "*"
          call:  append_tag
          code:  |
              function append_tag(tag, timestamp, record)
                 new_record = record
                 new_record["my_env"] = FLB_ENV
                 return 1, timestamp, new_record
              end

    outputs:
        - name:  stdout
          match: "*"

output:

test: [[1731990257.781970977, {}], {"my_env"=>{"A"=>"aaa", "C"=>"ccc", "HOSTNAME"=>"monox-2.lan", "B"=>"bbb"}, "rand_value"=>4805047635809401856}]

e.g: FLB_ENV['A'] access the string value aaa.

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[lecaros]

Docs PR #1523