feat: add etag and if-none-match support

[FrankYang0529]

related #3

> curl -i http://127.0.0.1:3000/content/hello-test.txt
HTTP/1.1 200 OK
cache-control: max-age=31536000
etag: 6926063833195007572
content-type: text/plain
content-length: 14
date: Sat, 25 Jun 2022 14:22:11 GMT

hello, world!

> curl --header "if-none-match: 6926063833195007572" -i http://127.0.0.1:3000/content/hello-test.txt
http://127.0.0.1:3000/content/hello-test.txt
HTTP/1.1 304 Not Modified
cache-control: max-age=31536000
etag: 6926063833195007572
content-type: text/plain
date: Sat, 25 Jun 2022 14:22:33 GMT

# change content in content/hello-test.txt and restart spin-fileserver

> curl --header "if-none-match: 6926063833195007572" -i http://127.0.0.1:3000/content/hello-test.txt
HTTP/1.1 200 OK
cache-control: max-age=31536000
etag: 11480734396890654158
content-type: text/plain
content-length: 4
date: Sat, 25 Jun 2022 14:23:39 GMT

123

> curl --header "if-none-match: 11480734396890654158" -i http://127.0.0.1:3000/content/hello-test.txt
http://127.0.0.1:3000/content/hello-test.txt
HTTP/1.1 304 Not Modified
cache-control: max-age=31536000
etag: 11480734396890654158
content-type: text/plain
date: Sat, 25 Jun 2022 14:23:51 GMT

[lann]

Thanks!