Add standard API for "modifying" httpx._urlparse.UNRESERVED_CHARACTERS

[Matiiss]

The need arises from having to use the $ character in URL params. But, of course, depending on the use case, this may be necessary for other characters as well.

For example:

response = httpx.get(url="mysite.com/api/db", params={"$filter": "..."})

The URL here will be encoded and the $ will become %24, but the endpoint expects a literal $.

Currently, my best solution has been to do this:

import httpx
from httpx import _urlparse as httpx_urlparse  # noqa

httpx_urlparse.UNRESERVED_CHARACTERS += "$"
response = httpx.get(url="mysite.com/api/db", params={"$filter": "..."})

But, as one might notice, this requires using some internal modules which is not ideal.
Thus, I would propose adding new API that would allow this to be configured by the user. Or integrate it into existing API, such that safe characters can be passed on to percent_encoded from the higher-level API, such as httpx.get