Support array index operator `[]` for CI/CD input interpolation
### Problem to solve
Currently, when using array-type inputs in CI/CD pipelines, users can only reference the entire array. There is no way to access individual elements of an array input during interpolation, which limits the flexibility of array inputs.
### Proposal
Introduce bracket notation with zero-based indexing to access individual elements of array inputs during interpolation using the `$[[ ]]` syntax.
**Example usage:**
```yaml
spec:
inputs:
supported_versions:
type: array
default:
- '2.0'
- '1.0'
- '0.1'
---
job:
script:
# Outputs: 'Latest version is 2.0'
- echo 'Latest version is $[[ inputs.supported_versions[0] ]]'
```
**Additional capabilities:**
- **Chaining with dot notation** for nested values: `$[[ inputs.servers[0].host ]]`
- **Multi-dimensional arrays**: `$[[ inputs.matrix[0][1] ]]`
- **Compatibility with interpolation functions**: `$[[ inputs.items[0] | truncate(0,5) ]]`
### Limitations
- Only non-negative integer indices are supported (0, 1, 2, ...)
- Negative indices are not supported
### Error handling
Clear error messages for:
- Index out of bounds
- Attempting to index non-array values
- Invalid index format
issue