rpc: Return block time in getblockchaininfo

Summary:

Return tip time in getblockchaininfo, for some use cases this can save a call to getblock.

Backport of core#22407.

Depends on D12550.

Test Plan:

./test/functional/test_runner.py rpc_blockchain

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12551