Add Table::describe

[texodus]

This PR "replaces" Table::validate_expressions with Table::describe, which validates a complete ViewConfig against a table and reports the schema a View built from it would have, without creating one. The old call could only type-check expressions in isolation; everything else about a config (bad column names, bad windows, bad aggregates) only surfaced by building a View and catching the failure.

The actual Table::validate_expressions method still exists, so this change is only breaking in the sense that the Virtual Server API now requires implementing describe instead of just validate_expressions. Internally however, it now creates an entire blank ViewConfig to host the expressions.

For a config like:

const verdict = await table.describe({
     // Config here
});

Expect one of these return values:

Return value
{ expression_schema, view_schema }	view_schema equals View::schema() of the same config
{ expression_schema, expression_errors }	one or more expressions failed to compile, valid ones still report their type
{ config_error }	expressions are fine, the rest of the config is not

In addition to perspective-server, all virtual servers have been updated and dedicated tests added.