Feature request: Add directives to reference multiple template files

### Describe your idea/feature/enhancement

I wish there was a way to build/deploy same with multiple templates. As templates grow larger, it is hard to maintain all of the resources in a single file including the parameters, mappings and globals. I wish there was a way where we can directly reference another template file from a template file.

### Proposal

Having directives such as `!Include` where we can reference the other templates within a template so we can have separate template file for parameters, mappings etc.

template-parameters.yaml
```
AWSTemplateFormatVersion: '2010-09-09'
Parameters:
MyParameter1:
Type: String
MyParameter2:
Type: String
```
template-resources.yaml
```
AWSTemplateFormatVersion: '2010-09-09'
Resources:
MyFunction:
Type: AWS::Serverless::Function
Properties:
FunctionName: my-function
Runtime: nodejs14.x
Handler: index.handler
```
main-template.yaml
```
AWSTemplateFormatVersion: '2010-09-09'
Transform: 'AWS::Serverless-2016-10-31'

# Reference the parameters from template-parameters.yaml
Parameters:
- !Include template-parameters.yaml

Resources:
# Reference the resources from template-resources.yaml
- !Include template-resources.yaml

# Additional resources can be defined here.
```

Things to consider:
1. Will this require any updates to the [SAM Spec](https://github.com/awslabs/serverless-application-model)

### Additional Details