24825: cherrypick-2.0: distsql: clean up lookup join internal columns r=pbardea a=pbardea

Cherry-picks #24697 and makes a few changes from the original PR:
- passing `nil` as the flowCtx in`processorBase.init` call in joinreader.go
- the output of the `EXPLAIN (VERBOSE)` test in the `lookup_join` logic test

Fixes #24401.

The distsql physical planner was not properly forming the post processor
spec according to the internal columns of lookup joins.

For the hash and merge joiners, the internal columns depend on the
streamToColMap of its upstream nodes. However since the lookup join's
internal columns consist of the entire right side of table columns, the
planner must modify the post processor spec and various maps.

This simplifies code inside the lookup joiner.


Release note (bug fix): Fix a bug causing some lookup join queries to
report incorrect type errors.

Co-authored-by: Paul Bardea <pbardea@gmail.com>