Investigate reducing overhead caused by `HrefPrnFieldMixin`

[dralley]

The overhead of HrefPrnFieldMixin is surprisingly high, being actually significant (10-20%) of the total time required during uploading of RPM packages using the synchronous APIs, and perhaps in other circumstances also.

pulpcore/pulpcore/app/serializers/base.py

Lines 66 to 80 in f53976e

	class HrefPrnFieldMixin:
	"""A mixin to configure related fields to generate relative hrefs and accept PRNs."""
	def get_url(self, obj, view_name, request, *args, **kwargs):
	# Use the Pulp reverse method to display relative hrefs.
	self.reverse = _reverse(obj)
	return super().get_url(obj, view_name, request, *args, **kwargs)
	def to_internal_value(self, data):
	# Properly also handle PRNs as values by converting them to URLs first
	if data.startswith("prn:"):
	model, pk = resolve_prn(data)
	obj = model(pk=pk) if self.use_pk_only_optimization() else model.objects.get(pk=pk)
	data = self.get_url(obj, self.view_name, self.context.get("request"), None)
	return super().to_internal_value(data)

First noted in a prior PR: #6693 (comment)