HeaderUtils: more efficient handling of optional/exotic charsets

While fixing Default Serializer in #891 we've identified [1] an area of performance improvement in `HeaderUtils`.

In case `hasContentType()` doesn't match the exact content-type with charset, we fall back to a `String` compare or a more complex regex match. This means we're doing an `AsciiBuffer` (the optimized underlying ServiceTalk construct for headers) to `String` conversion and performing multiple passes over the `Buffer` to compare.

We should be able to create a method in the `CharSequences` utility that performs a prefix with optional suffix match in a single pass, leveraging `ByteProcessor` on `AsciiBuffer` and avoid both both: `String` conversion and regex matching.

[1] https://github.com/apple/servicetalk/pull/891#issuecomment-564207308

Start by tracing HeaderUtils.hasContentType() and the CharSequences utility, then inspect how AsciiBuffer and ByteProcessor are used for header comparisons. Done means optional or exotic charsets are matched in a single pass without converting to String or using regex matching, with the relevant tests updated or added.

Written by the indexing model from the issue text.