Add `ParserConfig` accessors, convenience methods, and request parsing options

[tgnottingham]

Add missing ParserConfig accessors, convenience methods for configuring request and response parsing simultaneously, and request parsing options that bring request parsing configuration up to parity with response parsing configuration.

[tgnottingham]

Hi @seanmonstar, sorry to ping, but do you think there are any deal-breakers with the concept of this PR? The purpose was mainly to add parsing options that were present for responses but missing for requests (allow_spaces_after_header_name_in_requests and allow_obsolete_multiline_headers_in_requests).

I could see the addition of the methods that configure request parsing and response parsing simultaneously being undesirable, for example, and I can remove if so.