CXX-3077 BSON Binary Vector accessor, sub_binary builder

[mdbmes]

This is a basic implementation of CXX-3077, adding bsoncxx support for the BSON Binary Vector specification.

The main contribution in this PR is to add the bsoncxx::vector namespace, and especially the bsoncxx::vector::accessor template. A vector accessor can wrap any validated b_binary value as read-only, or a mutable accessor can be obtained during append.

General binary items can now be created and edited in-place during an append, using a new sub_binary basic builder.

Bumps the Libmongoc version to an unreleased pre-2.0.0 snapshot, to get access to new Libbson features: bson_append_binary_uninit, bson_vector_*_const_view_init, and bson_vector_*_binary_data_length. Validation for existing and newly allocated vectors has been delegated to Libbson, to reuse as much of its testing as possible. The actual vector accessor template in bsoncxx has been implemented in a way similar to Libbson's vector views, but without a direct dependency.

This implementation is currently less comprehensive than the Libbson implementation for BSON Binary Vector. No support is included for conversion between BSON arrays and BSON vectors, and there are no functions for bulk element conversion. Instead, a standard iterator interface allows access to individual elements or element ranges.

The spec includes JSON tests that are similar to bson_corpus. As with bsoncxx's approach to bson_corpus, this PR chooses to delegate those tests to the underlying Libbson implementation.

[mdbmes]

Thanks for all the great suggestions.

[eramongodb]

Minor suggestions remaining; otherwise, LGTM!