Debug Query as SQL string.

[pravic]

Summary

Allow to print the current SQL query, whether it's bound or not. Useful for logging and diagnostics.

Note: I chose Debug over Display because the latter is too implicit and somebody might accidentally pass a query into a string.

Albeit, something like fn Query::into_string(self) -> String can be useful on its own - as a poor man's query builder:

let q = client.query("SELECT id from system.users WHERE name = ?");
let q = q.bind("default");

// store the query string for later 
let query_string = q.into_string(); // returns `q.sql.finish()` inernally

// then execute it:
client.query(&query_string).fetch();

closes #146

Checklist

Delete items not relevant to your PR:

• Unit and integration tests covering the common scenarios were added
• A human-readable description of the changes was provided so that we can include it in CHANGELOG later
• For significant changes, documentation in README and https://github.com/ClickHouse/clickhouse-docs was updated with further explanations or tutorials
• conflicts with Query parser: encode ?? as ? #154, this PR needs to be rebased on top of that

[loyd]

Query can easily include more than client + sql in the future and Debug (despite Display) shouldn't be so restrictive.

I think, we should leave Query { client: Client, sql: "blabla" } here and don't pretend it's a simple str-like struct.

Another way is to provide fn sql_display(&self) -> &impl Display which also covers into_string() but in more much reliable way for future changes

[pravic]

Another way is to provide fn sql_display(&self) -> &impl Display

@loyd Do I understand it right, that we:

• keep the impl Display for SqlBuilder in this PR
• remove impl Debug for Query
• instead, add fn Query::sql_display

Correct?

[pravic]

Also, sql_display or as_sql?

[pravic]

@loyd Removed Debug in f4da92b.

[loyd]

Only misleading Debug impl, all other changes are fine

[CLAassistant]

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