Build IPFS evidence fetcher and validator in grantstream-verifie

[teethaking]

In grantstream-verifier, write a service that fetches milestone evidence from IPFS by CID using the reqwest crate against a public IPFS gateway. Validate that the fetched content is non-empty, matches expected content type (JSON, PDF, image), and is reachable. Return a VerificationResult enum: Valid, Unreachable, InvalidFormat.

[nonchalanttee22-lgtm]

ill like to do this also