HTTPX: add send method

[lspgn]

Hello, Thank you for this library. In some cases documented by HTTPX, when using directly the send method, the bearer token is not passed. This change adds a method with a similar signature to request and stream with an auth parameter (those two can likely be removed too).

The only thing I have not fully tested is self.ensure_active_token(self.token) as it's in a non async function.

Thank you

What kind of change does this PR introduce? (check at least one)

• [x] Bugfix
• [ ] Feature
• [ ] Code style update
• [ ] Refactor
• [ ] Other, please describe:

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No

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• [x] You consent that the copyright of your pull request source code belongs to Authlib's author.

[lepture]

@lspgn thanks for your work. Maybe you need to add a test case for send method.