feat: http pin/add

[koivunej]

Continued from #152 by @saresend cc #11 .

Closes #152.

This only adds the pin/add per current impl which passes the tests which don't verify anything. I am inclined to merge this in as is since the next PR is likely going to be a larger one.

Checklist (can be deleted from PR description once items are checked)

• New code is “linted” i.e. code formatting via rustfmt and language idioms via clippy
• There are no extraneous changes like formatting, line reordering, etc. Keep the patch sizes small!
• There are functional and/or unit tests written, and they are passing
• There is suitable documentation. In our case, this means:
  • Each command has a usage example and API specification
  • Rustdoc tests are passing on all code-level comments
  • Differences between Rust’s IPFS implementation and the Go or JS implementations are explained

[ljedrz]

LGTM
bors r+

[bors]

Build succeeded:

• ci