statement: Add selective data keypairs as storage

[vatsa287]

This PR allows to have selective data as storage in the chain-state.
This feature allows to have individual fields of digest in a document, with a maximum of 25 k:v's.
The key is custom, and value must be a digest of 32 length.

This implementation allows to check for availability of individual fields of a document has been anchored or not instead of whole document itself. This is crucial when verification is done using a non-digital copy as a reference digest.

Updates:

• register: Optional argument selective_data to add.
• update: Optional argument selective_data to update existing selective_data.
• update_selective_data: Update just the selective_data.

• remove_selective_data: Remove existing selective_data to free up space entry usage.

[vatsa287]

PTAL @smohan-dw @amarts @NiranjanAP