Helm - allow loading all river files in a folder

[fculpo]

PR Description

This PR implements this cli feature: #5228

When using a user provided configMap containing multiple river files, files are mounted to /etc/agent, but the chart only allowed to load a single entrypoint river. This behaviour, even with modules, makes overrides or unloading components in some clusters difficult when using a common configuration.

This PR enables loading all river files in /etc/agent (a la terraform). Configmap can then be overridden in some env by not loading some files or only modifying the needed one.

Which issue(s) this PR fixes

Fixes grafana/alloy#228

Notes to the Reviewer

PR Checklist

• CHANGELOG.md updated
• Documentation added

[CLAassistant]

All committers have signed the CLA.

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