Description

This PR adds optional WAL-G runtime settings under BackupConfig.spec.walg.

Previously, several WAL-G runtime parameters were only configurable through hardcoded defaults in NewConfigWithDefaults(), for example WALG_COMPRESSION_METHOD, GOMAXPROCS, TOTAL_BG_UPLOADED_LIMIT, WALG_FAILOVER_STORAGES_*, and WALG_PREVENT_WAL_OVERWRITE.

This makes operational tuning harder because optimal values may depend on database size, CPU limits, S3/MinIO performance, network limits, WAL generation rate, and backup/restore requirements.

With this change, users can configure these settings declaratively through a BackupConfig manifest, for example:

spec:
  walg:
    compressionMethod: brotli
    uploadConcurrency: 16
    uploadDiskConcurrency: 8
    networkRateLimitBytesPerSecond: 536870912

All new fields are optional. Existing behavior is preserved when `spec.walg` is not set. Existing top-level fields remain supported for backward compatibility.

Internal integration settings such as `PGHOST` and `PGUSER` are intentionally not exposed.

No related issue.

## Type of change

- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
- [x] Documentation update
- [ ] CI/CD or infrastructure change

## How Has This Been Tested?

Tested locally with:

```sh
go test ./api/v1beta1 ./pkg/walg
make generate
make manifests

• Unit tests
• Integration tests
• Manual tests

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules