multi: sharding key update improvements + bug fixes

[levkk]

• refactor: move sharding key update into its own module
• BREAKING: sharding key updates require a transaction started by the user instead of starting one automatically
• feat: support RETURNING clause in UPDATE that changes the sharding key
• fix: ErrorResponse message incorrectly parsed severity field
• feat: double-check we actually need to do a multi-step key update when the key isn't actually changed by reading the row from the database

This is common in Rails:

UPDATE orders
SET user_id = $1, amount = $2, created_at = $3,
/* [..every other model field..] */
WHERE id = $1

The sharding key doesn't actually change (user_id), the ORM just puts it into the query anyway.

[codecov]

Codecov Report

❌ Patch coverage is 93.64659% with 96 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/frontend/router/parser/value.rs	56.92%	28 Missing ⚠️
...frontend/router/parser/rewrite/statement/update.rs	96.31%	24 Missing ⚠️
.../frontend/client/query_engine/multi_step/update.rs	90.60%	17 Missing ⚠️
pgdog/src/frontend/router/parser/tuple.rs	17.64%	14 Missing ⚠️
...tend/client/query_engine/multi_step/test/update.rs	98.62%	5 Missing ⚠️
...c/frontend/client/query_engine/multi_step/error.rs	0.00%	3 Missing ⚠️
pgdog/src/frontend/router/parser/query/update.rs	93.75%	2 Missing ⚠️
pgdog/src/backend/server.rs	95.23%	1 Missing ⚠️
pgdog/src/frontend/client/query_engine/query.rs	92.30%	1 Missing ⚠️
pgdog/src/frontend/router/parser/insert.rs	88.88%	1 Missing ⚠️

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