Fix 3089 Allow credentials to be set via topology configuration

[antoinekh]

Fixes #3089
Fixes #2146
Fixes #2009

Summary

Allow credentials to be set via topology configuration. Adds username and password fields at any level of the topology hierarchy, with a well-defined resolution order.

New topology fields

Credentials can now be configured at the defaults, kinds, or nodes level:

topology:
  defaults:
    username: admin         # applies to all nodes
    password: mypassword
  kinds:
    nokia_srlinux:
      username: srl-admin   # overrides defaults for SRL nodes
      password: srl-pass
  nodes:
    router1:
      username: router1-user  # overrides everything for this node
      password: router1-pass

Credential resolution order

Most specific to least specific:

node → group → kind → defaults → kind's hardcoded default

Changes

Core types (types/node_definition.go, types/topology.go, types/types.go)

• Added Username/Password fields to NodeDefinition (YAML-serializable)
• Added GetNodeUsername()/GetNodePassword() methods to Topology with hierarchy resolution
• Added Username/Password to NodeConfig — Password is excluded from JSON marshaling for security

Core config (core/config.go)

• createNodeCfg() now populates credentials from topology, falling back to the kind's hardcoded registry defaults when not set

Node kinds

• All 40+ node kinds updated to use n.Cfg.Username/n.Cfg.Password instead of the package-level defaultCredentials variable

Inventory generation (core/inventory.go)

• Ansible and Nornir inventories now use per-node resolved credentials

Schema & docs

• Updated clab.schema.json and docs/manual/nodes.md

Tests (types/topology_test.go)

• Added TestGetNodeCredentials covering all hierarchy levels

[antoinekh]

Used in lab for a couple of days, no issues

[sacckth]

Great contribution. I'll let our benevolent containerlab leader to comment but there are some things left.
I found an issue with a potential rabbit hole. There are certain NOSes that don't have explicit initial configuration that sets the password and username (e.g. SRLinux). For this case, we need to add templates to render the username and password accordingly.

Similarly, in some other NOSes, we explicitly set the password in the default configuration (e.g. SROS SR-SIM).

A compromise would be to set the password/user explicitly in the startup-config which I believe most of the people did already.

We should probably add a few Robot tests that perform checks using SSH using the "default" creds. I suggest to cover a few usecases with this new variable.

[antoinekh]

@hellt pipeline failed for GitHub ratelimit error 429. Nothing related to this code

[hellt]

Thanks. Will check it out when time permits.
In the meantime others can check out it from the branch - that would help to get some real world converage

[hellt]

Hi @antoinekh would you mind trying this adaptation of this PR where the credentials are put under the credentials container:

credentials:
  username: foo
  password: bar

hope it still works :D