Deprecate get_placeholder in favor of get_placeholder_sql

[Jacob Walls]

For consistency with other expressions that ​return sql and params separately (e.g. tuple[str, tuple]), we should deprecate get_placeholder for a get_placeholder_sql method that returns tuple[str, tuple] and adapt BaseSpatialField, get_geom_placeholder, and other call sites like Value.as_sql to make use of it.

[Simon Charette]

Made some progress. Once I got the full suite passing I'll add the missing tests and release note docs for the deprecation.

[Jacob Walls <jacobtylerwalls@…>]

In 1a8fd5cf:

Fixed #36727 -- Deprecated Field.get_placeholder in favor of get_placeholder_sql.

The lack of ability of the get_placeholder call chain to return SQL and
parameters separated so they can be mogrified by the backend at execution time
forced implementations to dangerously interpolate potentially user controlled
values.

The get_placeholder_sql name was chosen due to its proximity to the previous
method, but other options such as Field.as_sql were considered but ultimately
rejected due to its different input signature compared to Expression.as_sql
that might have lead to confusion.

There is a lot of overlap between what Field.get_db_prep_value and
get_placeholder_sql do but folding the latter in the former would require
changing its return signature to return expression which is a way more invasive
change than what is proposed here.

Given we always call get_db_prep_value it might still be an avenue worth
exploring in the future to offer a publicly documented interface to allow field
to take an active part in the compilation chain.

Thanks Jacob for the review.

[Jacob Walls <jacobtylerwalls@…>]

In d43bd46:

Refs #36727 -- Factored out _must_transform_value() in BaseSpatialOperations.