[FEATURE] Shorthand way to define `files` and `volumes`

Detailed description

Score is designed for developers to quickly and easily define what their workloads need to run. Following the principals of incrementally repealing complexity and focusing on developer experience, there are a number of properties that could have short hand versions.

This PR will focus on files and volumes.

A shorthand for:

files:
  /usr/local/conf/app:
    content: "Hello world"

Could be:

files:
  /usr/local/conf/app: "Hello world"

Same for volumes.

Context

This is part of the broader iniative to move at some point the Score spec from v1b1 to v1.

Possible implementation

• Update the files and volumes part of the spec to support this 3rd options for end users to define files and volumes
  • https://github.com/score-spec/spec/pull/198
• In score-go use this open PR's branch to support this part there
• In score-compose use this open score-compose's PR's branch to support this part alongside still supporting the 2 other existing ways.
• Same as right above for score-k8s
• Once everything is validated and tested above, let's approve all the PR and do the release in cascade accordingly

Additional information

Take similar approach as what was done in the past with this exact same files and volumes part of the Spec: https://github.com/score-spec/spec/pull/123.