Add location utilities for v4 relative paths #1123

Format v4 allows relative paths in metadata location fields, resolved against the table location (Paths in Metadata, apache/iceberg#15630). iceberg-cpp has LocationUtil::ParseScheme but nothing to tell absolute paths from relative ones, resolve relative paths, or relativize paths when writing. The v4 metadata, manifest and catalog work needs these first.

Proposal: add three helpers to LocationUtil, following Java's LocationUtil (apache/iceberg#16174, apache/iceberg#17807):

• HasScheme(location): strict RFC 3986 scheme detection, which is the spec's test for an absolute path.
• ResolveLocation(table_location, path): absolute paths are returned unchanged; relative paths are joined to the table location with /. There is no ./.. normalization and no URI parsing, so encoded characters are kept as they are.
• RelativizeLocation(table_location, path): returns the part after <table location>/ if the path is under the table location, otherwise the path unchanged.

One difference from Java: if the table location ends in /, no second separator is added. The spec already recommends that a table location not end in /, and S3 treats // as part of the object key.

Out of scope: using the helpers in metadata, manifests or catalogs, and adding a scheme to scheme-less paths in pre-v4 tables. Both are follow-ups. Tests cover every example in the spec's path resolution table, plus edge cases: trailing slashes, sibling prefixes such as /t vs /t2, different schemes, escaped characters, and relativize-then-resolve round trips. Part of #1107.