Provide Base Devcontainer.json Inheritance

[simonkurtz-MSFT]

Devcontainers are wonderful. They empower Codespaces to work so seamlessly. But when multiple configurations exist, so does redundancy. An example of this can be seen in the APIM Samples repo.

• Please consider adding support for devcontainer.base.json.
• The devcontainer.json files should then be able to use extends or similar.
• The .base.json format should indicate that this is not an executable definition (relevant for extensions & GitHub UI)

@bamurtaugh