feat: add environment variable support for mapping handlers

[Gmin2]

Description

Extends subql existing environment variable support from ts manifests to mapping handlers, allowing developers to access .env variables during indexing runtime.

Fixes #2471

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist

• I have tested locally
• I have performed a self review of my changes
• [] Updated any relevant documentation
• Linked to any relevant issues
• I have added tests relevant to my changes
• Any dependent changes have been merged and published in downstream modules
• My code is up to date with the base branch
• I have updated relevant changelogs. We suggest using chan

[Gmin2]

cc @stwiname

[stwiname]

This looks really good, only a couple of minor changes. :D

[Gmin2]

done with the changes @stwiname