Add more standalone fuzzing harnesses

[elopez]

This PR adds some new standalone harnesses that fuzz:

• Alt-Svc parsing
• Base64 encoding/decoding
• DoH decoding
• Date parsing
• (un)escaping

Some of the harness code includes (from CURLDIR) or copies bits from internal headers; there might be a nicer way to do that.

[cmeister2]

Immediate comment - I'm mildly terrified about adding another 2000 files to this repo, but I don't know if that's FUD or not.

[elopez]

For each harness there's a few "seed" cases with human-readable names, plus some more inputs obtained through fuzzing, with hash-like filenames. We can probably remove the latter and let oss-fuzz rediscover them, if you don't want all the files committed in the repository.

[cmeister2]

For each harness there's a few "seed" cases with human-readable names, plus some more inputs obtained through fuzzing, with hash-like filenames. We can probably remove the latter and let oss-fuzz rediscover them, if you don't want all the files committed in the repository.

I realise I didn't respond here - yes, please, if you could do that that would be great.