feat: added vnet dns name generation as db custom label

[nibrasohin]

Changes

• Adds a new internal label teleport.internal/vnet-dns-name that is automatically set on every database resource during CheckAndSetDefaults
• The label value is a deterministic, DNS-safe hash of the database name in the format db-{base32hex(sha256(name)[:8])}, will be used by VNet in future PRs to resolve database FQDNs

Manual Test Plan

Test Environment

• Ensure you have at least one database registered with your local Teleport cluster
• Build tsh from this branch: make build/tsh

Test Cases

• Run tsh db ls -v --format=json and verify every database has the label teleport.internal/vnet-dns-name with a value matching the format db-<13 char base32hex hash> (e.g., db-k0i03gnmb0tng)
• Verify the label is deterministic: restart the database service and run tsh db ls -v --format=json again — the vnet-dns-name value for each database should be unchanged
• Verify uniqueness: confirm that no two databases with different names share the same vnet-dns-name value
• Verify existing labels are preserved: any database that already had user-defined labels (e.g., env=dev) should still have those labels alongside the new vnet-dns-name label

Changelog: Added vnet dns name as database custom label to be used for db access support via vnet