Fix query encoding

[nik-localstack]

Motivation

While working on #18 this was observed as a bug.

Summary

• decode and re-encode intercepted query text using the negotiated client_encoding
• keep query interception working for non-UTF8 sessions instead of hardcoding UTF-8
• add an end-to-end proxy test that exercises a LATIN1 client connection through the proxy

Testing

• make start-pg-and-test

[bentsku]

LGTM, nice and neat fix, quite the overlook 😅

[bentsku]

I just realized that there are several places in LocalStack where we patch the interceptor, and use to_str which will use utf-8 by default, so this might need to be updated too 😅