fix(compute_ctl): Allow usage of DB names with whitespaces (#9919)

## Problem

We used `set_path()` to replace the database name in the connection
string. It automatically does url-safe encoding if the path is not
already encoded, but it does it as per the URL standard, which assumes
that tabs can be safely removed from the path without changing the
meaning of the URL. See, e.g.,
https://url.spec.whatwg.org/#concept-basic-url-parser. It also breaks
for DBs with properly %-encoded names, like with `%20`, as they are kept
intact, but actually should be escaped.

Yet, this is not true for Postgres, where it's completely valid to have
trailing tabs in the database name.

I think this is the PR that caused this regression
https://github.com/neondatabase/neon/pull/9717, as it switched from
`postgres::config::Config` back to `set_path()`.

This was fixed a while ago already [1], btw, I just haven't added a test
to catch this regression back then :(

## Summary of changes

This commit changes the code back to use
`postgres/tokio_postgres::Config` everywhere.

While on it, also do some changes around, as I had to touch this code:
1. Bump some logging from `debug` to `info` in the spec apply path. We
do not use `debug` in prod, and it was tricky to understand what was
going on with this bug in prod.
2. Refactor configuration concurrency calculation code so it was
reusable. Yet, still keep `1` in the case of reconfiguration. The
database can be actively used at this moment, so we cannot guarantee
that there will be enough spare connection slots, and the underlying
code won't handle connection errors properly.
3. Simplify the installed extensions code. It was spawning a blocking
task inside async function, which doesn't make much sense. Instead, just
have a main sync function and call it with `spawn_blocking` in the API
code -- the only place we need it to be async.
4. Add regression python test to cover this and related problems in the
future. Also, add more extensive testing of schema dump and DBs and
roles listing API.

[1]:
4d1e48f3b9
[2]:
https://www.postgresql.org/message-id/flat/20151023003445.931.91267%40wrigleys.postgresql.org

Resolves neondatabase/cloud#20869

compute_tools/src/pg_helpers.rs

@@ -6,6 +6,7 @@ use std::io::{BufRead, BufReader};
 use std::os::unix::fs::PermissionsExt;
 use std::path::Path;
 use std::process::Child;
+use std::str::FromStr;
 use std::thread::JoinHandle;
 use std::time::{Duration, Instant};
 
@@ -13,8 +14,10 @@ use anyhow::{bail, Result};
 use futures::StreamExt;
 use ini::Ini;
 use notify::{RecursiveMode, Watcher};
+use postgres::config::Config;
 use tokio::io::AsyncBufReadExt;
 use tokio::time::timeout;
+use tokio_postgres;
 use tokio_postgres::NoTls;
 use tracing::{debug, error, info, instrument};
 
@@ -542,3 +545,11 @@ async fn handle_postgres_logs_async(stderr: tokio::process::ChildStderr) -> Resu
 
     Ok(())
 }
+
+/// `Postgres::config::Config` handles database names with whitespaces
+/// and special characters properly.
+pub fn postgres_conf_for_db(connstr: &url::Url, dbname: &str) -> Result<Config> {
+    let mut conf = Config::from_str(connstr.as_str())?;
+    conf.dbname(dbname);
+    Ok(conf)
+}