[API Node] Subscribe "pending" transactions

### Description of new feature request
Add an api feature such that clients could subscribe for `pending` transactions.

### Is this a hard-fork (and height to be activated)
- [X] No

### Implementation plan
- [] Phase 1 description (PR number)
- [] Phase 2 description (PR number)
- [] Phase 3 description (PR number)

### Affected modules
- [] action
- [x] actpool
- [x] api
- [] blockchain
- [] blockindex
- [] blocksync
- [] chainservice
- [] config
- [] consensus
- [] db
- [] dispatcher
- [] ioctl
- [] p2p
- [] server
- [] state
- [] tools

Start by examining the actpool and API modules to find the existing transaction and subscription entry points. The issue provides no files, tests, or implementation phases; done would require a defined API for subscribing to pending transactions and corresponding verification, which must be clarified before implementation.

Written by the indexing model from the issue text.