feat: add Content-Length headers when writing to a sphere.

[jsantell]

First pass on this; we could also add length when linking, either the size of the pointer or 0

[cdata]

Just as a general comment, I want to call attention to #498 . There isn't a strict path dependency here, but I think the overall priority if #498 is higher, and that a more wholesome solution to computing Content-Length would arise from fixing it.

[github-actions]

Test flake analysis

status	platform	features	toolchain
🟢	ubuntu-latest	test-kubo,headers,rocksdb	stable
🟢	ubuntu-latest	test-kubo,headers	stable
🟢	windows-latest	test-kubo,headers	stable
🟡	ubuntu-latest	test-kubo,headers	nightly
🟢	macos-13	test-kubo,headers	stable
🟢	macos-13	test-kubo,headers,rocksdb	stable

Flake summary for ubuntu-latest, test-kubo,headers, nightly

     Summary [  41.129s] 177 tests run: 177 passed (1 flaky), 1 skipped
   FLAKY 2/5 [   1.806s] noosphere-ns dht::node::test::test_dhtnode_bootstrap

[cdata]

LGTM