Rust PostgreSQL connection client

[mmastrac]

Implements a basic Rust postgres connection library and re-uses its internal state machine to replace the guts of pgcon.

User-facing changes:

• Bugfix: EdgeDB will choose the smaller of server max_connections or the maximum number of connections defined for the user's current role to prevent hitting connection limits.
• Change: sslmode: SSL is always requested unless sslmode=disable. If the server returns S to indicate SSL support and then fails to accept the user credentials, we do not retry without SSL.
• Feature: Plain-text password authentication in backends is now supported, though discouraged.

Changes:

• New PGRawConn that behaves as a transport and fully connects to postgres before returning the transport to the protocol. This transport can then be used transparently and with zero overhead as it only uses Rust code for connection setup.
• New source_description parameter wherever we use connections directly or indirectly to assist with tracking down what particular part of the system requested a connection. This is not currently propagated into the parameters we send to the server, but we could do this in the future.
• Major improvements to DSN parsing, as well as moving all DSN code to Rust. There is now a thin shim in pgconnparams.py, but it delegates all logic to the Rust side of things.
• class CreateParamsKwargs(TypedDict) is used wherever we hold a bag of connection parameters to assist with typing.
• pgaddr has been removed from the EdgeDB server settings as it was not sufficient to connect to the backend. It has been replaced with a fully-rendered DSN. edgedb-rust will need to be updated to support this.

Testing strategies

DSN parsing: we have a full battery of DSN parsing tests that are compared against libpq. The tests that were previously implemented for pgconnparams have been migrated into this rust codebase. The tests used internally for postgres to test DSN parsing were also extracted and added to the library. On top of this, a number of "hard" examples were added to exercise the code.

Auth code: auth code is tested against a "capture" postgres that we inittb, boot up on a socket and test. Tests for each auth method (scram, md5, password) are tested with and without SSL.

DSN notes

The DSN parsing code behaves effectively the same as the previous version of the code with a few small differences:

• Backslash escaping/unescaping in pgpass and the connection string now fully match the behaviour of libpq
• A few additional warnings were added in cases where an explicit passfile was specified but cannot be read (we still don't warn for implicit passfiles).

Examples:

Parse a DSN:

cargo run --example dsn 'postgres://username@/database_name?host=path-to-socket'

Query a running server:

cargo run --example connect -- -U foo -P secret -u <<direct path to unix socket .s.PGSQL.5432>>

[fantix]

Epic! LGTM

(There're a few println!("Read:"); in the future connection Rust code; a kind reminder to replace them in the next PR)