feat: add support for specifying starting IP address for BMC and node allocation

[alexlovelltroy]

Checklist

• My code follows the style guidelines of this project
• I have added/updated comments where needed
• I have added tests that prove my fix is effective or my feature works
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Description

This pull request adds support for specifying the starting IP address for both BMC and node IP allocation, allowing users to reserve lower IPs in a subnet (e.g., for gateways or DNS) and begin allocation at a desired address. The implementation introduces new command-line flags, updates the allocation logic, and adds comprehensive tests and documentation.

Key changes include:

Feature: Start IP allocation at a specific address

• Added --start-ip flag to init-bmcs and --node-start-ip flag to discover, enabling users to skip a range of IPs at the beginning of the subnet for both BMC and node allocations. [1] [2] [3] [4]
• Updated allocation logic in internal/netalloc/ipam.go with a new ReserveUpTo method, which reserves all IPs before the specified start IP. This is used by both initbmcs.Generate and discover.UpdateNodes. [1] [2] [3] [4]

Documentation updates

• Enhanced README.md with usage examples and explanations for the new --start-ip and --node-start-ip flags, clarifying how to reserve IPs at the start of a subnet. [1] [2]

Testing

• Added unit tests for the new ReserveUpTo method and for generating BMC entries with a specified start IP, ensuring correct behavior and error handling. [1] [2]

Internal API changes

• Updated function signatures and calls in initbmcs.Generate and discover.UpdateNodes to accept the new start IP parameters, maintaining backward compatibility. [1] [2] [3]

These changes provide more flexible IP management and improve usability for network administrators.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

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