Implement shttp3 server example

[koflin]

Implementation of a shttp3 server example analogous to the shttp server example.

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This change is 

[marcfrei]

Reviewed 5 of 6 files at r1, all commit messages.
Reviewable status: 5 of 6 files reviewed, 3 unresolved discussions / 0 of 1 LGTMs obtained / 0 of 1 approvals obtained

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_examples/shttp3/server/README.md line 4 at r1 (raw file):

## Simple server example 
The server example `bin/example-shttp3-server` is analogous to the  [shttp simple server example](../../shttp/README.md#simple-server-example), except that it always uses HTTPS and thus requires a certificate and key.

remove extra ' '

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_examples/shttp3/server/main.go line 31 at r1 (raw file):

func main() {
	certFile := flag.String("cert", "", "Path to TLS server certificate for optional https")

remove

Code quote:

optional

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_examples/shttp3/server/main.go line 32 at r1 (raw file):

func main() {
	certFile := flag.String("cert", "", "Path to TLS server certificate for optional https")
	keyFile := flag.String("key", "", "Path to TLS server key for optional https")

remove

Code quote:

optional

[marcfrei]

@JordiSubira What do you think, should we add this shttp3 example or would it better to have this integrated into the existing shttp server example?

Reviewable status: 5 of 6 files reviewed, 3 unresolved discussions / 0 of 1 LGTMs obtained / 0 of 1 approvals obtained

[JordiSubira]

Both work for me, I do not have a strong opinion. We could also add perhaps a shttp3 client example in this PR, to have a complete H3 example.

Reviewable status: 5 of 6 files reviewed, 3 unresolved discussions / 0 of 1 LGTMs obtained / 0 of 1 approvals obtained