Add full Bowtie2 index and reference FASTA for host contamination testing

[Alexaz13]

This PR adds the necessary files to support nf-test testing of the bowtie2_align module with the PolyBio dataset. Specifically, it includes:

• A full Bowtie2 index (hg38.*.bt2 files) for the human genome.
• The matching reference FASTA file (GCA_009914755.4_T2T-CHM13v2.0_genomic.fna).

These files were added in accordance with nf-core guidelines for test datasets.

Purpose

These additions will allow us to:

• Perform host contamination filtering tests using real paired-end FASTQ data.
• Validate the functionality of the bowtie2_align module using human reference genome alignment.

Notes

• Files are hosted under polybio/host/ and polybio/ncbi_genome/ to maintain organization.
• The FASTA and index files match and were sourced from NCBI (T2T-CHM13v2.0).
• Large files were pushed in batches to avoid timeouts.

[411an13]

We shouldn't merge the test data to master. Each pipeline has a dedicated branch (and a special one for modules).