Added support for arbitrary Bioc SNPlocs tarballs (dbS > 156)

[sufyansuleman]

This pull request introduces a new dbSNP_tarball argument across several core functions—most notably in R/load_snp_loc_data.R, R/validate_parameters.R, and R/format_sumstats.R—to allow users to supply an arbitrary Bioc SNPlocs tarball (e.g. for builds ≥156) instead of being limited to the hard-coded 144/155 packages. When dbSNP_tarball is provided, load_snp_loc_data() automatically untars the archive, detects either a flat TSV or per-chromosome .rds files, and ingests them via data.table::fread() or readRDS() + rbindlist(). We verified that, after installing SNPlocs.Hsapiens.dbSNP156.GRCh37/GRCh38, calling format_sumstats(..., dbSNP = 156) works seamlessly without supplying a tarball, and that supplying both dbSNP = 156 and dbSNP_tarball = "" also correctly loads the same data from file. For older or unreleased builds (e.g. 157+), users can simply point to a downloaded tarball—even in the absence of an installed Bioc package—and the code will fall back to reading that archive. In testing on a toy summary-stats file, all downstream allele-flipping and RSID-imputation checks in R/check_no_rs_snp.R and R/check_no_snp.R completed successfully with build 156 both with and without the tarball. This enhancement preserves the original fallback behavior for 144/155, adds full flexibility for any future dbSNP release, and ensures backward compatibility.