Support custom VM IDs in create-l1 tool

## Problem

The `create-l1` tool hardcodes `constants.SubnetEVMID` when calling `IssueCreateChainTx`. This means the tool can only deploy Subnet-EVM chains — any custom VM (e.g. a DAML VM, MoveVM, or other custom AvalancheGo plugin) requires forking the tool.

## Proposed Solution

Add a `--vm-id` flag to `tools/create-l1` that accepts a CB58-encoded VM ID string. When provided, use it instead of `constants.SubnetEVMID`. Default behavior unchanged (SubnetEVM if flag is omitted).

Also add Ansible support for installing custom VM plugins on validator nodes:
- New variables: `custom_vm_id`, `custom_vm_binary_path`
- New task in `ansible/roles/avalanchego/tasks/main.yml` to copy the custom VM binary to the plugins directory

## Use Case

We have a custom AvalancheGo VM (`block.ChainVM` implementation) that runs DAML smart contracts natively. VM ID: `nNQ83pDiUEV1sobcMFWiJK1QZD38Rm56TC5SU2eKMqmnYZsdF`. We need to deploy this to Fuji using the existing avalanche-deploy infrastructure.

## Changes Required

1. `tools/create-l1/main.go` — Add `--vm-id` flag, parse with `ids.FromString()`, pass to `IssueCreateChainTx`
2. `ansible/roles/avalanchego/tasks/main.yml` — Add custom VM plugin installation task
3. `ansible/roles/avalanchego/defaults/main.yml` — Add `custom_vm_id` and `custom_vm_binary_path` variables
4. Documentation update

Start with tools/create-l1/main.go and trace the IssueCreateChainTx call, then inspect the avalanchego role defaults and tasks. Add the optional VM ID handling while preserving the Subnet-EVM default, install the configured custom plugin on validator nodes, and update the documentation to cover the new variables and flag.

Written by the indexing model from the issue text.