String conversion API needs extensions

[jtv]

For the 8.0 release it would be useful to extend the string conversion API with additional parameters:

• Array conversions need encoding knowledge which may not always be available — see Allow parsing arrays without connection #841.
• It'd be helpful if we could pass a std::source_location into a conversion — see Use std::source_location in exceptions. #923.

Obviously though I don't want to break compatibility on the string conversion API on short notice. We'll need a smooth transition process.

Some ideas...

• We could add a separate API versioning scheme to the string conversions. But it's tedious and ugly and I don't want to maintain a list of API versions. Also, there's a chance that we won't even need the scheme again in the future.
• Or we could add optional string_traits fields for "these functions accept an encoding" and "these functions accept a source_location" and make them default to false. (If I can't make that work, they could be stand-alone constants). It feels like double specification though.
• Or we could use nasty template trickery to suss out a conversion function's signature, and figure out which arguments to pass. There's some ad-hockery there (because there are several conversion functions to support), but we already do at least part of this work in pqxx::internal::args_f.

The new parameters would have to have defaults for compatibility. That's not always safe for encodings, but I think it's feasible to enforce this at compile time.

[jtv]

Looks like it's actually reasonably simple to do:

1. Use decltype(pqxx::internal::args_f(pqxx::string_traits<T>::function)) to get a tuple representing the parameter types for a member function of pqxx::string_traits<T>.
2. Extract individual parameter types using std::tuple_element_type<index, params_tuple_type>.
3. Remove irrelevant qualifications using std::remove_cvref_t<param_type>.
4. Compare the result to an expected type using std::is_same<stripped_type, expected_type>.

We can then specialise code for different API versions using if constexpr.

[jtv]

I've got much of this written up and am getting close to merging it into the 8.0 working branch.
However I still need to add an optional encoding parameter.

[jtv]

I've just added the encoding (group) parameter. I'm not making this a fancy abstract type after all; just the enum should work.

The encoding group and the calling std::source_location live in the same conversion_context struct. A bit of an awkward marriage, but it works well enough and (I think this is more important than it might seem) it doesn't add a lot of text to all those function signatures.