Add different types of validation methods to blob

[cmwaters]

Besides Validate, the node needs two more validate funcs. https://github.com/celestiaorg/celestia-node/blob/540192259c144ccbd24e45e34616a41389232a51/share/namespace.go#L114-L129.

We need this so we can restrict to only blobs(excluding reserved) and to only real data(excluding parity)

Originally posted by @Wondertan in #68 (comment)

[cmwaters]

I had a look and found that there already exists functions that cover this purpose, for example we have an IsReserved() method to check if it's part of the primary or secondary reserved namespaces. And I will add a IsUsableNamespace in #87

[cmwaters]

Closing this as completed