Provide block ID as context in form so form_alter hooks can tell which block's form they have

Problem/Motivation

Say I have two Configurable Views Filter Blocks. Beyond hiding and showing certain fields, I would also like to alter the form differently for these two blocks.

Currently, if I attempt to alter the form through a form alter hook, my hook will be invoked for each instance of the block, but I can't seem to tell which block I am altering from that hook. There does not seem to be any context to latch onto telling me which block instance of the form I am working with.

It would be nice if I had something I could use to tell, in a form alter hook, which Configurable Views Filter Block's form I am working on.

Latching onto what was altered by this module (eg. fields or buttons that were hidden) to determine form instance is not an option because this module's hiding/altering seems to happen after form_alter hooks are invoked.

Steps to reproduce

1. Create two Configurable Views Filter Blocks atop the same view, Block A and Block B.
2. Implement hook_form_alter() or similar to alter the form these blocks expose.
3. Load a page that displays the two blocks and see, via xdebug or similar, that the hook_form_alter() function is run once for each block, but that there's nothing you can use in the hook_form_alter() function to tell which block you're working on.

Proposed resolution

Add block ID context into the form such that form alter hooks can tell which Configurable Views Filter Block they are working with.

Remaining tasks

MR + review.

User interface changes

None.

API changes

Addition of block ID as context in form array.

Data model changes

None.