Replace forked net parsers with regexp hacks

[danwinship]

Proof-of-concept, since I keep mentioning we could do this if we wanted. I'm not sure we do want to, but...

Note that we only use the regexps if the initial attempt at parsing fails, so we only take the regexp hit on invalid or legacy inputs.

/kind cleanup
/cc @aojea @thockin

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[danwinship]

/hold

[liggitt]

I like the idea of being able to not just tolerate an IP using old style parsing, but actually transform it into the acceptable version, so that we could conceivably scrub existing data and stop accepting leading 0s via the API at some point in the future.

as nits, if we were going to do this for real, I'd want to split the IP and CIDR cases so we match exactly what we intend to for each scenario, and probably try to factor out the matcher for the leading-zero number to make the overall structure easier to see:

var (
  num = `0*(0|[1-9][0-9]*)`
  dot = `\.`
  
  leadingZeroIP = regexp.MustCompile(`(^|:)` + num + dot + num + dot + num + dot + num + `$`)
  leadingZeroIPReplace = `$1$2.$3.$4.$5`

  leadingZeroCIDR = regexp.MustCompile(`(^|:)` + num + dot + num + dot + num + dot + num + `/` + num + `$`)
  leadingZeroCIDRReplace = `$1$2.$3.$4.$5/$6`
)

I'd also consider whether we wanted to use these cleaners externally to detect a field was relying on this (so we could warn) or to do the transform (so we could normalize or clean up existing or incoming data)

[danwinship]

to detect a field was relying on this (so we could warn)

KEP-4858 already took care of all of this (for non-CRD types). kubernetes/kubernetes#122550 split IP/CIDR validation into IsValid{IP,CIDR} for new fields vs IsValid{IP,CIDR}ForLegacyField, for older fields, and kubernetes/kubernetes#128786 added warnings when you're using a value with 0s in a legacy field.

As for transform/clean-up, that's been an ongoing discussion, most recently in #328 (comment) (which is the proximate cause of me filing this PR).