Improve speed (& logging) of file QC checks

[selkamand]

When processing 288 samples (each with SNV, SV, and CNV entries), the log suggests it takes over 4 minutes from checking manifest headers to counting samples, leaving users without any progress updates or understanding of the delay.

[2025-03-26T08:57:09Z INFO scarscape::utils] Found Manifest with 4 columns: sample,snv,cnv,sv
[2025-03-26T09:01:04Z INFO scarscape::utils] Manifest describes 288 unique samples

During this non-logged period, we validate manifest files for downstream analysis by checking file extensions, ensuring VCFs are bgzipped and indexed. We should make 2 changes to the codebase

1. Log the ongoing checks to inform users of the current activity:
   info!("Checking all files described in manifest are suitable for analysis");
2. Optimize these validations for speed — investigate whether the bgzip compression check is causing the delay

[selkamand]

Added info log statement in 0d590a8