use custom input labels for connected block focus

[maribethb]

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Description

if a block is focused, and the input it is connected to has a custom label, then also display the input's label text in the block's aria label.

if there is no custom label, do NOT fall back to using the field row. in that case, don't include anything about the connected input (just like we have today)

this only applies when a block that is connected to another block's input is focused (for full-block field blocks, the field element is technically the thing that is focused and in that case we should still include the custom input label). it does not apply when reading out the block that contains the custom-labeled input itself.

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