Add AlmaLinux 8 custom AMI support

### Feature request

Add first-class `almalinux8` support for customer-provided ParentImage and CustomAmi workflows. No official ParallelCluster image is requested.

### Current behavior

The image-build operating-system detector reads `/etc/os-release` and rejects an AlmaLinux 8 parent before the ParallelCluster cookbook can run, even though supported Enterprise Linux 8 variants share most package and service behavior.

### Proposed scope

- Add an explicit `almalinux8` operating-system value.
- Accept AlmaLinux 8 in image-build detection and image metadata.
- Require an explicit customer-provided parent/custom AMI.
- Extend schema, validators, serialization, documentation, and tests.
- Coordinate the platform implementation with `aws-parallelcluster-cookbook`.

### Reproduction

Run `pcluster build-image` with an official AlmaLinux Foundation AlmaLinux 8 x86_64 AMI as `Build.ParentImage`. The default ParallelCluster Image Builder component exits during OS validation after identifying `almalinux.8.10`.

I am preparing focused CLI/ImageBuilder and cookbook changes with unit tests and a public AlmaLinux integration build.

Start with the pcluster build-image path and its /etc/os-release operating-system detector, then trace the related schema, validators, serialization, and tests named in the issue. Check the aws-parallelcluster-cookbook coordination point and verify that AlmaLinux 8 custom ParentImage and CustomAmi workflows accept explicit customer images without requesting an official ParallelCluster image.

Written by the indexing model from the issue text.