Add support for arbitrary JSON blocks

[tgvashworth]

As Slack introduces new block types, this change allows users to include any block type by specifying its JSON representation directly.

Create blocks from raw JSON for testing new Slack features before library support, and for copy-pasting from Block Kit Builder.

Usage

// Copy JSON from Block Kit Builder
contextActionsJSON := `{
    "type": "context_actions",
    "block_id": "feedback",
    "elements": [...]
}`

// Create block (preserves all fields)
block, err := slack.BlockFromJSON(contextActionsJSON)
// or use MustBlockFromJSON() in tests

// Mix with regular blocks
msg := slack.NewBlockMessage(
    slack.NewDividerBlock(),
    block,
    slack.NewSectionBlock(...),
)