feat: update chisel-slices shcema to accomodate essentials from v3

[lczyk]

update the schema for chisel slices to accommodate new essential format introduced in chisel 1.4.0

• https://documentation.ubuntu.com/chisel/latest/reference/chisel-releases/slice-definitions/#essential

old:

essential:
  - base-files_tmp
  - libc_libs

new:

essential:
  base-files_tmp:
    arch: [arm64]
  libc_libs:

since its impossible to determine which version is being uses just from the slice file, the type is a union of list (old) and object (new).

[lczyk]

@cjdcordeiro fyi/wdyt?

[madskristensen]

Thanks