Update block validation endpoint to return the processing time

[obycode]

In addition to reporting if the block is valid, the node should also report the block cost. EDIT: it already reports the block costs.

In addition to reporting if the block is valid, the node should also report the processing time.

See #5434.

[jferrant]

Does it not already do this?

stacks-core/stackslib/src/net/api/postblock_proposal.rs

Line 146 in bac1b07

pub cost: ExecutionCost,

[obycode]

Does it not already do this?

Updated to add the processing time instead.