Enable shared yum cache for mock builds

## Summary

Add support for sharing the yum/dnf package cache across mock builds of the same platform and architecture. This avoids redundant package downloads when multiple builds run for the same platform/arch combination, improving build performance.

## Changes

### Shared yum cache (`use_shared_yum_cache`)

When `use_shared_yum_cache: true` is set in a platform's `data.mock` config, the build node generates a `cache_key` in mock.cfg based on the platform name and target architecture:

```
config_opts["plugin_conf"]["yum_cache_opts"]["cache_key"] = "almalinux-kitten-10-x86_64"
```

This tells mock's yum_cache plugin to share the cached packages across all builds matching that key, instead of maintaining separate caches per chroot.

### Yum caching options (`keepcache`, `metadata_expire`)

Platform configs can now set `keepcache` and `metadata_expire` in `data.yum` to control yum.conf caching behavior inside the chroot. AlmaLinux-Kitten-10 is configured with `keepcache: true` and `metadata_expire: 60`.

### Nested dict support in MockConfig

`MockConfig.render_config_option` now supports two-level nested dictionaries, enabling direct setting of `plugin_conf` options without resetting existing plugin defaults.

## Pull Requests

- AlmaLinux/albs-build-lib#5 — nested dict support in `render_config_option`, `keepcache` and `metadata_expire` parameters in `YumConfig`
- AlmaLinux/albs-node#221 — shared yum cache handling in build node
- AlmaLinux/albs-web-server#1210 — enable `use_shared_yum_cache`, `keepcache`, and `metadata_expire` for AlmaLinux-Kitten-10

Merge order: albs-build-lib → albs-node → albs-web-server