chore: parse escaped double colon (\\:) example struct tag

[GusAntoniassi]

Describe the PR
Replaces the regular string splitting by a ":" made during the "example" tag parsing so that it will not split escaped double colons, allowing string examples containing a double colon to be escaped using double backlashes.

Relation issue
#1401

Additional context
Since the core regexp library cannot suport negative lookaheads, I had to combine a regular expression with a loop to capture all the ocurrences and append them to an array.

[sdghchj]

I see your purpose is to make the map value contain colons.
But I wonder why not use a light mode to fix it, for example, use strings.IndexRune to find the first colon and split the string into two parts, a key and a value.

[GusAntoniassi]

Hello @sdghchj, initially I went with this approach to allow escaped double colons in the key value, but thinking better about it it does not make much sense and I can't think of a valid use case for it.

I've refactored the code using strings.SplitN to split at the first ocurrence of a :, this makes the change much simpler and from my tests it won't be necessary to escape the remaining double colons in the value, so for example the struct from my issue would work without any changes:

type Foo struct {
  Attributes map[string]string `example:"bucket_arn:arn:aws:::foobar-bucket"`
}