Add template name defaults for SAM commands

### Describe the feature

From related PR (https://github.com/aws/aws-cli/pull/3444):
> SAM strives to make deployment of serverless applications to AWS as
> easy as possible. To make it even easier as it is right now, this commit
> adds defaults for the template file names the cloudformation package
> and cloudformation deploy commands take.
>
> The default values for the template names are taken from the
> [SAM Howto](https://github.com/awslabs/serverless-application-model/blob/master/HOWTO.md). If there are better fitting values I'm of course
> open to adjusting them.
>
> Together with https://github.com/aws/aws-cli/pull/3040 this commit would allow running
> cloudformation package without the need to provide any arguments.

### Use Case

^See above

### Proposed Solution

https://github.com/aws/aws-cli/pull/3444

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CLI version used

any

### Environment details (OS name and version, etc.)

.

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For more context: the SAM CLI looks like for anything matching the pattern template `[yaml|yml|json]` for both its package and deploy command. If the AWS CLI were to introduce defaults we would want to copy what the SAM CLI does in order to avoid creating any new differences that end users have to keep track of. For now customers should 👍 this issue if they want to see this implemented.