feat: partial sharding support for PREPARE and EXECUTE

[levkk]

Prepared statements

Correctly store statements sent with PREPARE in the global cache. We use the whole query string with data type specifications as key, while anonymizing the statement name. This is equivalent to using Parse as cache key.

PREPARE __stmt_1(int) AS SELECT $1;

becomes

PREPARE __pgdog_template_name(int) AS SELECT $1;

It's then dynamically rewritten to

PREPARE __pgdog_1(int) AS SELECT $1

at execution time.

This is because it's kinda hard to parse the data types out of the string and we need a unique key identifying the prepared statement in the cache.

Sharding

Handle EXECUTE by extracting parameters and passing them to our query router. This works with I think most of our sharding features, i.e., direct-to-shard and cross-shard queries.

TODOs

1. Comment-based routing isn't supported. This is because we parse and deparse the original statement to put it in the prepared statement cache, which erases the comment.
2. Query rewrites, e.g., insert split, don't work yet.

Notes for reviewers

1. Most of the diff in client/prepared_statements module is from moving stuff around. There is some diff for handling PREPARE, that's where I would focus the review.
2. Simulated messages sent by Server now get processed through the server state machine; this was necessary for correct state management for handling PREPARE statements; seems to have no bad side effects.

[codecov]

Codecov Report

❌ Patch coverage is 93.84010% with 47 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pgdog/src/net/messages/prepare.rs	56.25%	14 Missing ⚠️
pgdog/src/frontend/router/parser/params.rs	84.50%	11 Missing ⚠️
...gdog/src/frontend/prepared_statements/statement.rs	86.95%	6 Missing ⚠️
pgdog/src/frontend/router/parser/query/prepare.rs	85.36%	6 Missing ⚠️
.../src/frontend/client/query_engine/test/replicas.rs	75.00%	4 Missing ⚠️
...gdog/src/frontend/prepared_statements/cache_key.rs	84.61%	2 Missing ⚠️
...router/parser/rewrite/statement/simple_prepared.rs	93.54%	2 Missing ⚠️
pgdog/src/net/protocol_message.rs	60.00%	2 Missing ⚠️

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