Add optional iconv-extra layer for legacy character encodings

[kolos]

Summary

Adds an optional iconv-extra Lambda layer containing the glibc conversion modules that are present in the Bref build image but omitted from the slim runtime image.

Applications can attach the layer when they need legacy character encodings such as Windows-1250, without increasing the size of every Bref runtime.

Fixes: brefphp/bref#2145

Usage

provider:
    environment:
        GCONV_PATH: /opt/lib/gconv

functions:
    web:
        layers:
            - ${bref-extra:iconv-extra-php-83}

GCONV_PATH must be set before PHP starts so glibc can discover the modules under /opt/lib/gconv.

Reproduction

Without the layer, this conversion fails in the Bref PHP 8.3 runtime:

iconv('WINDOWS-1250', 'UTF-8', hex2bin('f5fbc1c9'));

With the layer and GCONV_PATH=/opt/lib/gconv, it returns őűÁÉ.

The issue also reproduces through Microsoft ODBC/PDO_SQLSRV when fetching a SQL Server varchar column using a Windows-1250 collation. The query fails with Unicode conversion failed without the layer and succeeds with it.

Size impact

Measured for PHP 8.3/x86-64:

• Uncompressed: 8,332,066 bytes (about 7.95 MiB)
• Compressed ZIP: 3,803,538 bytes (about 3.63 MiB)

Keeping this optional avoids adding that size to applications that only use UTF-8/Unicode encodings.

Validation

layer=iconv-extra php_versions=83 make test
layer=iconv-extra php_versions=83 make layers