relation: add support for selectAll()

[akphi]

• [x] relation: Expose relation::distinct
• [x] relation: support usage of select() with no params, i.e. equivalent to selectAll()

[github-actions[bot]]

Test Results

  1 022 files  ±0    1 022 suites  ±0   1h 23m 3s :stopwatch: +27s 12 153 tests +1  12 063 :heavy_check_mark: +1  90 :zzz: ±0  0 :x: ±0  17 671 runs  +9  17 581 :heavy_check_mark: +9  90 :zzz: ±0  0 :x: ±0 

Results for commit 3562fb1a. ± Comparison against base commit 72c4f10f.