feat(coredhcp/coresmd): introduce hostname patterns

[synackd]

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Description

Introduce node_pattern, bmc_pattern, and domain configuration keys to allow custom hostnames to be set for nodes and BMCs (respectively) that are stored in SMD. The default behavior before was effectively to set node_pattern=nid{04d}, meaning that the default format is nid followed by a three-zero-padded node ID (e.g. nid001). The optional domain key allows a domain to be specified that is appended to hostnames, making them fully qualified.

Here's a snippet of documentation of these keys from examples/coredhcp/coredhcp.conf:

# CUSTOM HOSTNAMES:
#
# For node_pattern and bmc_pattern, custom hostname patterns can be
# specified to dynamically generate custom hostnames for BMCs and nodes in
# SMD.
#
# Hostname Placeholder Syntax:
#
#   {Nd} - Zero-padded NID where N is the number of digits
#          Examples: {04d} -> 0001, 0042, 1234
#                    {02d} -> 01, 42, 99
#                    {05d} -> 00001, 00042, 12345
#   {id} - Component xname (e.g., x3000c0s0b0n0 for nodes, x3000c0s0b1 for 
#          BMCs)
#
# Literal text can be mixed with placeholders:
#
#   "compute-{04d}" -> compute-0001, compute-0042
#   "node-{id}"     -> node-x3000c0s0b0n0
#   "dev-s{02d}"    -> dev-s01, dev-s42
#
# CUSTOM DOMAINS:
#
# If 'domain' is specified, a custom domain will be appended to hostnames
# generated using the custom hostname mechanism above, making generated
# hostnames fully-qualified.
#
# For instance, for the following configuration:
#
#   - coresmd: ... node_pattern="{id}" bmc_pattern="{id}" domain=cluster.local
#
# {id} will be expanded to the node/BMC's xname and the domain cluster.local
# will be appended. For example:
#
#   BMC hostname:  x3000c0s0b1.cluster.local
#   Node hostname: x3000c0s0b1n0.cluster.local

Type of Change

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

---

For more info, see Contributing Guidelines.

[synackd]

Creating this draft PR to build testable container images. Before this is ready for review/merge, #37 should be merged since this is based off of that, and it should be rebased onto the new main.