Add static IP address as a build feature

[svrotter]

In some cases it might be advantageous to set a static IP address for Stabilizer e.g. if it is used in an embedded environment that doesn't support DHCP or troubleshooting problems with DHCP negotiation.
This PR add this functionality as a build feature.

[ryan-summers]

It occurs to me that we could actually do this entirely based on the presence of some input environment variable instead of using a cargo-feature. E.g. use some default UNSPECIFIED IP if nothing's provided on the STATIC_IP variable, otherwise use the provided IP.

[svrotter]

Couldn't set it to UNSPECIFIED somehow, it always panics then. Maybe that's my incompetence but setting it to 0.0.0.0 and checking for is_unspecified () worked

[ryan-summers]

bors try

[bors]

try

Build succeeded:

• doc
• compile (1.57.0)
• compile (1.57.0, pounder_v1_1)
• compile (stable, pounder_v1_1)
• style
• compile (stable)
• HITL Run Status

[ryan-summers]

Looking good so far, only further changes:

1. Add an entry to the CHANGELOG
2. Either update the docs in book/ to mention static IP configuration options or spawn an issue so we can add it later

[jordens]

LGTM
But do update the book or file an issue.

[ryan-summers]

bors r+

[bors]

Build succeeded:

• compile (1.57.0)
• compile (1.57.0, pounder_v1_1)
• compile (stable)
• compile (stable, pounder_v1_1)
• doc
• HITL Run Status
• style