Optimize costly operations for pekko-http server

[kciesielski]

When running performance tests, I found some operations that take some noteworthy CPU time:

• sttp.tapir.server.pekkohttp.PekkoServerRequest.acceptsContentTypes() 2.4%
• sttp.tapir.server.pekkohttp.PekkoToResponseBody.parseContentType(String) 3.53%
• sttp.tapir.server.pekkohttp.PekkoModel$.parseHeadersOrThrowWithoutContentHeaders(HasHeaders) 2.34%

These operations run header parsers from sttp-model underneath, which are expensive mostly because they use regular expressions, like Accepts.parse.