enable client-side ssl certificates for oauth2 session

[pingz]

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

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

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

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

(If no, please delete the above question and this text message.)

---

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

[lepture]

Thanks

[schmocker]

can i set the ssl cert for all requests made by a flask client? I'm using resp = remote_app.get('me', verify=False) but it would be nice to set the ssl cert only once for each client.