BSON-as-JSON indented output

[vector-of-bool]

This changeset adds options to bson_as_json that supports prettier-formatted output. Helpful when debugging differences between large documents side-by-side. These options are only exposed in the private API at the moment. It works like:

• level_indent - If NULL, all behaves as previously. If not-null, this string is inserted at the beginning of the line N times for a line at depth N.
• initial_indent - This is inserted before the opening brace.
• subsequent_indent - This is inserted before the level_indents at the beginning of every line (except for the first line).

(This was useful while debugging csfle oddities.)