add `--sparse` option to `get`, `pin add` subcommands, for broken file

#### Type: Enhancement

#### Description:
Sometimes I have a file in my local repo that's missing a single block, that's nowhere on the network either. When I `ipfs get `, it stalls at the position where that block is in the file, forever. Media files usually tolerate a single missing block just fine, so I'd like a `--sparse` option that would simply skip unavailable blocks and leave it unwritten in a sparse file or nulled in filesystems that don't support sparse files. *Unavailable* would mean that the block stalls (0bps downstream) longer than X seconds or takes Y% (~1000%) times longer than the other blocks to retrieve. Maybe X and Y could be configurable in the daemon's settings.

Similarly, a `--sparse` or maybe `--partial` option to `ipfs pin add` would skip over the missing blocks and pin the rest, while still returning some non-zero error, because the entire file could not be pinned.

The other motivation for this besides playing media files is that when I lose pieces of a file and want to recover them from a torrent, I have no easy way to get the trailing good blocks out of the ipfs datastore. I presume the API allows me to fetch each block individually and write it into the file myself, but I think this facility should be part of the CLI. Likewise, when pinning a file, I sometimes want to transfer over as much as possible in the background, while working on recovering the last piece to complete the pin.

Start by reading the existing `ipfs get` and `ipfs pin add` command paths and their handling of unavailable blocks. Clarify how sparse output, timeout thresholds, configuration, and partial pin errors should work. Done means both subcommands have an agreed `--sparse` or `--partial` behavior, with appropriate tests and documented non-zero failure handling.

Written by the indexing model from the issue text.