p2p/http: cache json wellknown mappings in the .well-known handler

[Jorropo]

We are expecting this to be vastly read dominated, caching this makes sense.

[Jorropo]

2023-08-29T10:34:18.3807361Z === RUN   TestSingleCandidate
2023-08-29T10:34:18.3896197Z     autorelay_test.go:137: 
2023-08-29T10:34:18.3901183Z         	Error Trace:	D:/a/go-libp2p/go-libp2p/p2p/host/autorelay/autorelay_test.go:137
2023-08-29T10:34:18.3904419Z         	Error:      	Condition never satisfied
2023-08-29T10:34:18.3906688Z         	Test:       	TestSingleCandidate
2023-08-29T10:34:18.3908211Z --- FAIL: TestSingleCandidate (10.13s)

Unrelated CI flake