aconfig: introduce cache

Introduce the Cache struct to represent parsed and verified aconfig and
override content. Most commands in aconfig will work of an existing
cache file, eliminating the need to re-read the input every time.

Restructure main.rs to use clap to create a proper command line
interface with support for sub-commands. main.rs is responsible for
parsing the command line, performing disk I/O and calling the correct
subcommand implementation (in commands.rs).

To simplify unit tests, subcommands never perform explicit I/O; instead
they only work with Read and Write traits.

Also add dependencies on clap, serde and serde_json.

Bug: 279485059
Test: atest aconfig.test
Change-Id: Ib6abf2eabd264009804f253874b6fba924fc391b