Relative Path Support In Table Spec

### Proposed Change

Currently, Iceberg uses absolute URIs for file paths, requiring metadata rewrites when a table is moved.

We want to introduce Relative path which is to achieve "Zero Rewrite Relocation", significantly simplifying table management and portability, with new tables defaulting to relative paths as primary goal.

More details in the document attached.

### Proposal document

https://s.apache.org/iceberg-spec-relative-path

### Specifications

- [x] Table
- [ ] View
- [ ] REST
- [ ] Puffin
- [ ] Encryption
- [ ] Other

Start with the linked proposal document and the Table specification item in the checklist. The issue names no source files or tests, so first determine the implementation scope for relative paths and which parts of the proposal apply to tables. Done should mean the Table portion of the proposal is addressed and its behavior is verifiable.

Written by the indexing model from the issue text.