Provide a warning when `add`ing a CAR

[rvagg]

When the user does a filecoin-pin add foo.car then it's likely not what they want to be doing (it may be, but I can't think of why). We should instead present a clear warning to them and suggest import is what they want.

• We should probably proceed anyway cause it's what they asked for and we don't do interactive-mode for add
• We could fail as long as we provide a --force or similar override for this case, cause it could be legitimate

Using decoder.readHeader(reader) (from a @ipld/car/decoder import) should be enough to do this ~gracefully. If it doesn't error then it's almost certainly a CAR.

[siddharthbaleja7]

@rvagg can you assign this issue to me?

[rvagg]

Assigned @siddharthbaleja7; but you'll need to investigate this issue properly and understand it first. Please don't just throw it at AI without understanding the output and being critical of it before you open a PR. Thanks!