feat: support macro

[sigoden]

# <aichat-config-dir>/macros/test-function-calling.yaml
steps:
  - .role %functions% what is the weather in Pairs and Berlin?

# <aichat-config-dir>/macros/within-agent.yaml
variables:
  - name: agent
  - name: args
    rest: true
    default: What can your do?
steps:
  - .agent {{agent}}
  - '{{args}}'

close #1077

[milanglacier]

Thanks for the great work, this is a really exciting feature. Will definitely try it!