Fix GH-8576: Bad interpretation of length when char is UTF-8

For columns of type `SQL_TEXT`, Firebird does not properly report the
actual column length, but rather only the maximum colum

Fix GH-8576: Bad interpretation of length when char is UTF-8

For columns of type `SQL_TEXT`, Firebird does not properly report the
actual column length, but rather only the maximum column length, so for
multi-byte encodings like UTF-8, such columns may have trailing
spaces. We work around that by treating such columns as `SQL_VARYING`
when we ask the server to describe the colum, what yields the desired
results.

Given that this is a work-around, and may break code which expects the
results with trailing spaces, we target "master" only.

Closes GH-8926.

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