[5.x] Use default value from field config

[duncanmcclean]

This pull request makes it so when there's no raw value for a field (eg. data saved for the field in an entry's YAML file), Statamic will now augment the default value for the field, if one is provided. This means that you no longer need to re-save all entries after setting the default value for a field.

This PR is targeting master (Statamic 5) since changing the behaviour is a breaking change.

Fixes #8804.

[jasonvarga]

I was about to merge this but then realized we have Field::defaultValue() and Fieldtype::defaultValue(). Maybe we should think about all the relevant spots and make sure it all works as expected.

Maybe it's just that $this->fieldtype->config('default') should get changed to $this->fieldtype->field()->defaultValue(). Tests would need to check for fieldtypes that provide their own default values.

[jasonvarga]

Just dumping thoughts here. You don't need to make any changes. I'll loop back around to this.

Somewhere that this is a little iffy is in any entry listing.

My Something field has a default value, but you don't see it anywhere. I'd expect to see it.

This is because we deal with just values, not augmented values:

cms/src/Http/Resources/CP/Entries/ListedEntry.php

Line 61 in 6ac62b4

$value = $extra[$key] ?? $this->resource->value($key);

I don't think Entry::value() should be inheriting the default values though, since it's supposed to just be a way to see what values have been explicitly set.

So maybe we just change that ListedEntry::values() method to get the default fallbacks.

Within templates, GraphQL, and the API, it all seems good. 👌