[receiver/postgresql] Update explainQuery client method to also include context

[kartikgola]

Description

Noticed this issue (while working on #49352) where explainQuery API in client.go doesn't take in context unlike the rest of the interface functions, and uses background context instead.
This can be problematic since if the context is cancelled, the explainQuery API will not honor it and could keep on going. Some problems that can arise -

1. EXPLAIN query would ignore the scrape timeouts and cancellations
2. Collector shutdown won't be honored by the receiver
3. Breaks client interface established conventions
   As part of the fix, I added the context back in the API and updated the function.

There's a small caveat with DEALLOCATE PREPARE though; In the original code it was using global context and was unbounded, I have now added it with a timed out context since DEALLOCATE PREPARE must be called even if the parent context is cancelled to stop polluting the shared connection pool.

Testing

Unit tests

Documentation

N/A

Authorship

• I, a human, wrote this pull request description myself.

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