Add username validation for names

[emcfarlane]

Adds a regex pattern match for usernames "^[a-z][a-z0-9-]*[a-z0-9]$". Must start with a lowercase letter, only consist of alpha numerics or hypens and doesn't end with a hypen. Bumped the min length to 2 as the RE2 syntax doesn't support lookback syntax.

Error on failure will be:

validation failed: validation error:
 - name: value does not match regex pattern `^[a-z][a-z0-9-]*[a-z0-9]$` [string.pattern]

Should we instead use a CEL expression for better errors?

(buf.validate.field).cel = {
  id: "username",
  message: "value must start with a lowercase letter and contain only letters, numbers, or hypens and cannot end with a hypen",
  expression: "this.matches('^[a-z][a-z0-9-]*[a-z0-9]$')"
}

validation failed: validation error:
 - name: value must start with a lowercase letter, contain only letters, numbers, or hypens and cannot end with a hypen [username]

[stefanvanburen]

This seems reasonable to me. (Would be a great place for reusable custom validation!)

I slightly prefer the CEL with the better error description, but 🤷‍♂️ - does converting between the two cause a buf lint error? (Feels like they shouldn't if they ultimately result in the same expression?)

[jchadwick-buf]

JFYI, you can make the constraint work with single-character owner names with a minor adjustment.

^([a-z]|[a-z][a-z0-9-]*[a-z0-9])$

In practice, it is unlikely any BSR will ever have owner lengths of 1 character, but if you wanted the semantically correct regex for that case, this should be it. (Using a look-behind would be unnecessarily complex.)