Allow feature authors to customize the names of the main executables for features.

This proposal is an enhancement idea to the current [dev container features proposal](https://github.com/devcontainers/spec/blob/main/proposals/devcontainer-features.md).

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`install.app` and `install.file` properties for feature authors to control the name of the main executable of their features, also allows them to add some parameters and to create features in ways different than shell scripts.

(Previously discussed in https://github.com/devcontainers/spec/pull/27#pullrequestreview-961566955.)

_Originally posted by @chrmarti in https://github.com/devcontainers/spec/pull/48#pullrequestreview-1014797101_