database backend: support @ in role name

[frco9]

Description

closes #31613

I'm trying to write a templated policy to allow users to have access to a list of roles they can use.

Instead of writing a policy rule per database role, the templated policy I want looks like:

path "dummy-database-engine/static-roles/{{identity.entity.aliases.<<mount accessor>>.name}}-*" {
  capabilities = ["create", "update"]
}

In my case, the mount is an OIDC mount, and the entity alias name with the default plugin is the user_claim, which in my case is the user email

Without this PR, the @ character is invalid, so I cannot create a database role that will match this policy rule.
And I get the following error :

│ URL: PUT https://vault.customdomain.io/v1/dummy-database-engine/static-roles/[email protected]
│ Code: 404. Errors:
│ 
│ * 1 error occurred:
│ 	* unsupported path
│ 
│ 
│ 

[vercel]

@frco9 is attempting to deploy a commit to the HashiCorp Team on Vercel.

A member of the Team first needs to authorize it.

[hashicorp-cla-app]

All committers have signed the CLA.