Allow developers to extend StreamValue

[ababic]

I have encountered a couple of situations where I wanted to customise the behaviour of StreamValue:

1. In a version of StreamField that doesn't include block definitions in migrations, I wanted to override the logic in StreamValue.to_python() that strips out values for blocks that are not recognised.
2. In a client project, I wanted to add functionality to StreamValue to autogenerate unique IDs for heading blocks and use them (and the heading text) to generate a structured 'minimap' for users.

However, using anything other than the built-in StreamValue class is currently quite difficult, due to various assumptions in the codebase. This PR aims to address these assumptions, allowing the value class to be more easily substituted.

[squash-labs]

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Test this branch here: https://ababiccustom-streamvalue-suppo-xetco.squash.io

[zerolab]

in __init__ we have:

if isinstance(block_types, Block):
    # use the passed block as the top-level block
    self.stream_block = block_types

but StreamField really expectes BaseStreamBlock (which inherits from Block). I wonder if the check should change to BaseStreamBlock, or perhaps this to use getattr.

I couldn't find any tests of what happens if you try to initialize StreamField with a non-StreamBlock block. Perhaps I am overthinking this

[gasman]

I couldn't find any tests of what happens if you try to initialize StreamField with a non-StreamBlock block. Perhaps I am overthinking this

This isn't currently supported (but might be in future, once #9746 lands).

[gasman]

Thanks @ababic! A few suggestions, nothing too major. (Unfortunately #9746 / #11850 have conflicted with this, but hopefully the changes are actually for the better as they mean more of the logic is contained in wagtail.blocks.stream_block rather than leaking into wagtail.fields.StreamField...)

[gasman]

We should define this as part of the inner Meta class, like BaseStructBlock does - that way, subclasses can define this in their own Meta class, which is probably a more natural way of working than passing it to the StructBlock constructor.