Add a default DOH service provider.

[KS-OTO]

I have submitted a new reply on the issue that specifically collects DOH service providers, but I haven't received a reply. For this reason, I have created a new issue. Looking forward to a reply.
#363 (comment)

[jyyi1]

Thanks, let me paste the original content here:

This is a suggestion regarding the addition of a DNS provider:
Currently, multiple DNS providers are built into intra for users to freely choose from. However, all the built-in DNS providers mentioned above are limited for some objective reasons, and basically none of them can provide a good experience in the network environment in mainland China; as a result, users in mainland China find it difficult to directly enjoy the convenience of the APP function design with the help of the built-in providers in intra, and instead need to configure the custom DNS provider function independently, but this is a decline in the user experience.
Therefore, it is recommended that you can build my maintained DoH service for users in mainland China into the intra APP: 18bit DNS.
The public welfare free DNS service of 18bit was created in March 2022 and has been running smoothly until now, with more than 940 million DNS requests resolved daily;
Moreover, the 18bit official has been recommending to users to use the DNS service provided by 18bit with the help of the intra APP and providing the hosted download service and usage tutorial of the intra APP since its establishment;
Today, I hope to upgrade the experience of this part of users once again. For this reason, I raise this issue, hoping that the built-in DNS provider of intra can include the 18bit DNS service to improve the experience of users in mainland China;
18bit website: https://www.18bit.cn/
DOH line: https://doh.18bit.cn/dns-query