Reorg the stage exec struct

[steiler]

This is a slight reorg to #2238.

The structure is now as follows:

nodes:
    frr1:
      kind: linux
      stages:
        configure:
          exec:
              - command: <command>
                target: <host / container>                optional, with a default of container
                phase: <on-enter / on-exit>            optional, with a default of on-enter

[codecov]

Codecov Report

Attention: Patch coverage is 84.93151% with 11 lines in your changes missing coverage. Please review.

Project coverage is 51.74%. Comparing base (6859e39) to head (161cc97).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
clab/dependency_manager/dependency_node.go	79.48%	6 Missing and 2 partials ⚠️
types/stages.go	90.90%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2241      +/-   ##
==========================================
- Coverage   52.12%   51.74%   -0.38%     
==========================================
  Files         172      172              
  Lines       12714    16699    +3985     
==========================================
+ Hits         6627     8641    +2014     
- Misses       5174     7143    +1969     
- Partials      913      915       +2     

Files with missing lines	Coverage Δ
types/topology.go	69.26% <100.00%> (-2.70%)	⬇️
types/stages.go	69.59% <90.90%> (+0.09%)	⬆️
clab/dependency_manager/dependency_node.go	84.21% <79.48%> (-3.45%)	⬇️

... and 160 files with indirect coverage changes