dag: Support Go template syntax

**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`](https://masterminds.github.io/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:
```yaml
name: example
params:
NAME: "World"
GREETING: "Hello"
steps:
- name: simple-greeting
command: "{{.GREETING}}, {{.NAME}}!"
```

Using sprig functions:
```yaml
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:
```yaml
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.