feat: support cloud storage URIs in CSV samplesheet path_prefix

[ikhrustalev]

Summary

Closes #484

Adds support for cloud storage URIs (gs://, s3://, etc.) in the path_prefix column of CSV samplesheets.

Problem

SamplesheetParser.resolvePath() treats any path not starting with / as relative and resolves it against the samplesheet's parent directory. A path_prefix like gs://bucket/path/sample gets mangled into a local path.

Additionally, Nextflow's cloud path objects (e.g. GcsPath) don't support string concatenation for suffix appending, causing downstream failures even if path resolution is fixed.

Fix

• Detect URI schemes (gs://, s3://, etc.) via regex and return them early as plain strings, bypassing relative path resolution
• Returning as strings (rather than Nextflow path objects) ensures suffix concatenation works correctly downstream

Test plan

• CSV samplesheet with gs:// path_prefix works on GCP
• CSV samplesheet with s3:// path_prefix works on AWS
• Local relative and absolute paths still resolve correctly