feat: support optional .npmrc mount for dynamic plugin installation

[Fortune-Ndlovu]

Description

This PR introduces support for optionally mounting a local .npmrc file into the install-dynamic-plugins container in RHDH Local, enabling developers to install dynamic plugins from private NPM registries without modifying the compose.yaml file.

What’s Included

• Adds a conditional .npmrc mount in compose.yaml, targeting the plugin installer container
• Supports an optional NPM_CONFIG_USERCONFIG env var, injected only if .npmrc is present
• Updates fixes.sh to copy the mounted .npmrc to the correct location (~/.npmrc) for plugin install
• Adds a new start.sh helper script that:
  • Accepts a path to .npmrc (defaulting to $HOME/.npmrc)
  • Copies it into the repo
  • Exports the appropriate env var
  • Starts the RHDH Local stack with podman-compose
• README updated with clear instructions on how to use .npmrc for private plugins

Benefits

• Zero manual edits to Compose files required
• Seamless developer experience for plugin testing behind a corporate/private registry
• Backward-compatible with existing workflows
• Gracefully handles the absence of .npmrc no mount, no env, no errors

Which issue(s) does this PR fix or relate to

• Resolves [RHDHBUGS-200](https://issues.redhat.com/browse/RHDHBUGS-200)
• Implements support described in [RHIDP-4263](https://issues.redhat.com/browse/RHIDP-4263)

PR acceptance criteria

• Tests
• Documentation

How to test changes / Special notes to the reviewer

[Fortune-Ndlovu]

cc: @rm3l , @kadel

[rm3l]

I think it will be better for this to be built on top of the changes in #34

[kadel]

@Fortune-Ndlovu can you please fix your git config? Your commits have my-name <my-email> as an author. I've noticed this in some of the commits in your other PR.