Enhancement: Add "dns_label" or "hostname_label" validator for single-label DNS validation

[sayem314]

Currently, the hostname_rfc1123 tag validates a full hostname which can include multiple labels separated by dots (e.g., sub.domain.com). However, many modern use cases, such as validating a subdomain part in a multi-tenant application (e.g., [xxx].example.com) or Kubernetes resource names, require validating a single DNS label that strictly forbids dots.

Proposed Change:
Add a new baked-in validation tag, such as dns_label or hostname_rfc1123_label, that enforces the following RFC 1123 rules for a single part:

• Characters: Only alphanumeric (a-z, A-Z, 0-9) and hyphens (-).
• Start/End: Must start and end with an alphanumeric character.
• Length: 1 to 63 characters.
• No Dots: Strictly forbids the . character within the string.

regexp=^[a-zA-Z0-9]([a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?$

Use Case:
Validating a single subdomain prefix before appending it to a base domain in code:

type Tenant struct {
    // Current hostname_rfc1123 would allow "illegal.sub.domain"
    // We need something that only allows "legal-sub"
    Subdomain string `validate:"required,dns_label"` 
}