feat: Add API and Node Support for Retrieving Blobs by Commitment without height

[walldiss]

Implementation ideas

Rollups that use external ordering require the ability to retrieve blobs solely by their commitment and namespace, without needing to provide height. We need to introduce API support in the blob module for “get by commitment” and a corresponding “has check.”

Requirements:

• Add a new API endpoint in the blob module to retrieve blobs only by commitment.
• Add a “has” method to check blob existence by commitment.
• Update BN storage to index blob commitments, enabling efficient lookup and retrieval.

Include necessary API changes and storage-layer updates to support this functionality.

security note: this method will not be able to privde non-inclusion proof

Context: Discussed in relation to supporting rollups that rely on non-Celestia ordering with @cmwaters

[walldiss]

It is open to discussion whether it should be solely by commitment or commitment+namespace. Technically both are possible to implement and it is up to the users if they are fine if blob with same commitment could exist in different namespace