Allow db only remove foreign key to work as database only

[s-cooper18]

Remove foreign key for database was previously unable to apply as it could not find the name of the field within the state. This is modified to be able to function with only a name and model provided, even if it is not on the state by handling the error and allowing the ForeignKeyManager to be populated with an optional field value.

It is noted that the Field value is still required in order for other operations to occur however.

Allows the following to work:

    operations = [
        migrations.SeparateDatabaseAndState(
            database_operations=[
                operations.SaferRemoveFieldForeignKey(
                    model_name="portfolio",
                    name="my_favourite_foreign_key",
                ),
            ],
        ),
    ]

[github-actions]

Coverage Report Results

Name	Stmts	Miss	Branch	BrPart	Cover
src/django_pg_migration_tools/indexes.py	9	0	0	0	100%
src/django_pg_migration_tools/management/commands/migrate_with_timeouts.py	138	0	20	0	100%
src/django_pg_migration_tools/operations.py	473	0	92	0	100%
src/django_pg_migration_tools/timeouts.py	73	0	30	0	100%
tests/django_pg_migration_tools/management/commands/test_migrate_with_timeouts.py	126	0	0	0	100%
tests/django_pg_migration_tools/test_indexes.py	33	0	0	0	100%
tests/django_pg_migration_tools/test_operations.py	1839	0	0	0	100%
tests/django_pg_migration_tools/test_package.py	3	0	0	0	100%
tests/django_pg_migration_tools/test_timeouts.py	75	0	0	0	100%
TOTAL	2769	0	142	0	100%

6 empty files skipped.

[Nickmika]

I am no expert in this library, but the test and the SQL generated looks like I would expect. Thank you. Left one comment. 👍

[Nickmika]

Do state only operation - has already been removed from state

Should this read "Do database only operation"?

[s-cooper18]

🧀 It feels a little clunky to have this still being optional, but causes the add_fk_field to fail if the field is not added. Especially with having to do asserts from within the function in order to confirm that field is actually populated.

Alternative I can think of is having one manager where it is optional and another where it is not, but this seems like possible overkill

[marcelofern]

Cool! I've left a comment about the CHANGELOG.md change
but please do

[marcelofern]

Not sure if this is valid markdown list item? I.e., isn't it missing a padding on the second and third lines like the other entries below?

[s-cooper18]

Seems to be valid markdown, but makes it a "quote" instead. Have pushed a fix anyway