[IO] Access list of file extensions of seqan3 I/O input classes

[Irallia]

While solving https://github.com/seqan/sharg-parser/issues/42 we would like to to access the file extensions via:

#include <seqan3/io/sequence_file/input.hpp>
#include <seqan3/io/sequence_file/output.hpp>

sharg::input_file_validator validator1{seqan3::sequence_file_input<>::file_extensions};

std::cerr << validator1.get_help_page_message() << '\n';

[joergi-w]

I found that this is currently possible with the following code:

seqan3::input_file_validator<seqan3::sequence_file_input<>> validator1{};

(found here: test/snippet/argument_parser/validators_input_file_ext_from_file.cpp)