[torreypayne]

Adds Commit#send_message(queue, key, payload, deliver_time: nil) and Commit#ack_message(queue, key, ignore_not_found: false). Each adds a Send or Ack mutation and returns the key.

• key is one message key: a scalar, or an array of the key's column values for a composite primary key. (Commit#delete reads an array as several keys instead.)
• Payloads are converted like column values, so binary payloads are passed as StringIO.
• The methods aren't named send because that would override Object#send.

Part 2 of 5, split out of #260. Transaction support comes in part 3.

[quartzmo]

LGTM. The unit tests pass on Ruby 3.2 through 4.0 locally. Two small notes:

• The description says "Keys work like Commit#delete", but for an Array they don't: delete "T", [1, 2] deletes two keys, while send_message "Q", [1, 2], ... sends one composite key (1, 2). The method docs are right; it's only the description, which does matter if it ends up in the squash commit.
• FYI, the new doctest examples fail locally with unmocked method :create_session, but so do 321 of 365 examples on main: the doctest mocks still stub batch_create_sessions. CI doesn't run doctest, so I think you can consider this out of scope for this PR. Just mentioning it.

[torreypayne]

Thanks for the careful read. I fixed the description: key is a single message key, so an array is one composite key, while delete takes a list of keys (see key_set). Agreed that the doctest failures predate this PR; I'll leave them for a separate one.

[quartzmo]

LGTM, pending fully green CI.