dag: Support Go template syntax

[yohamta]

Overview
Enhance Dagu's configuration files with Go template support, enabling dynamic content generation and variable substitution using Go's template/text package. Additionally, incorporate the rich set of utility functions from sprig to provide powerful template operations.

Features

• Full Go template syntax support in most configuration fields
• Access to all sprig template functions (string operations, math, lists, etc.)
• Variable substitution across configuration
• Environment variable support
• Built-in helper functions

Example Usage

Basic variable substitution:

name: example
params:
  NAME: "World"
  GREETING: "Hello"
steps:
  - name: simple-greeting
    command: "{{.GREETING}}, {{.NAME}}!"

Using sprig functions:

name: templating-example
params:
  USER: "alice"
  ENV: "production"
steps:
  - name: conditional-step
    command: |
      {{- if eq .ENV "production" -}}
      deploy --production --user {{.USER | upper}}
      {{- else -}}
      deploy --staging --user {{.USER | lower}}
      {{- end -}}

  - name: date-based-task
    command: "backup-{{now | date \"2006-01-02\"}}.tar.gz"

Environment variables can be accessed using the env function:

steps:
  - name: env-aware
    command: "deploy --token {{env "API_TOKEN"}}"

This enhancement would make Dagu configurations more flexible and powerful, allowing for dynamic content generation based on variables, environment, and runtime conditions.