Add PgTsVector type and implement deserialization

[encalypto]

This commit introduces the PgTsVector, PgTsVectorEntry types. We also add an implementation of FromSql<TsVector, Pg> for PgTsVector for deserialization.

Two new tests were also added to check deserializing tsvectors with and without lexeme positions.

[prkbuilds]

Hi @encalypto , Thanks for the PR, the implementation looks correct. I just have one issue I wanted clarity on -

Will the lexeme text will always end with null terminator? If not I suggest handling the lexeme.pop() with some logic to check if it ends with null terminator.

[weiznich]

Will the lexeme text will always end with null terminator? If not I suggest handling the lexeme.pop() with some logic to check if it ends with null terminator.

I think the format specifies that this is always the case. We nevertheless could still end up in a situation where this is not the case, for example if we get a truncated network package. I don't think we should care about these cases too much, as we would fail afterwards anyway.

[weiznich]

Thanks for submitting this, looks good for me