SAM: enable sam build cache by optionally building entire lambda template

**Is your feature request related to a problem? Please describe.**

If you specify a specific resource within `sam` with cache enabled

```shell
sam build MyHandler --cached
```

`sam` will not persist a `build.toml` which means it can not make use of cached builds on subsequent builds.

VSCode always requires you to select a specific resource and it always provides it to the `sam build` command and thus we cannot make use of cache.

**Describe the solution you'd like**

An option to build entire template rather than just such that we can make use of cache builds for faster workflow.

**Additional context**

This behaviour of sam isn't documented so I have linked to the code where its disabled

https://github.com/aws/aws-sam-cli/blob/ac47ffcc72fb690f3ecb4421f1a3e76246964002/samcli/lib/build/app_builder.py#L207

From sam comments:

```
is_building_specific_resource : boolean
Whether customer requested to build a specific resource alone in isolation,
by specifying function_identifier to the build command.
Ex: sam build MyServerlessFunction
```

Start with the linked samcli/lib/build/app_builder.py#L207 code and compare the behavior of `sam build MyHandler --cached` with a whole-template build. Trace how the AWS Toolkit for VS Code supplies the selected resource, then define the option so a whole-template build can persist and reuse build.toml; verify the requested cached workflow end to end.

Written by the indexing model from the issue text.