fix: allow dynamic storages in ExternalLocation.from_name

[lmmx]

1. Dynamic Storage Backends

Description

As discussed in #311 (discussion) I am building a prototype of the phash: storage protocol for
ExternalLocation so we can write external("phash:") and have a file be saved to
content-addressable storage using a perceptual hash rather than a cryptographic one, and this will
effectively serve as deduplication as well as a novel way to tolerate inexact matches in file
content in a snapshot.

To add this we discussed changing the validation from hardocing hash and uuid strings to instead
validate against state().all_storages whose keys would provide the set of possible storage
protocol prefixes.

This PR does so, and changes the error message from

ValueError: storage has to be hash or uuid

to

ValueError: storage 'phash' is not registered. Available: 'hash', 'uuid'

available = ", ".join(map(repr, state().all_storages.keys()))
raise ValueError(
    f"storage '{storage}' is not registered. Available: {available}"
)

• I comma separate the repr of the string keys rather than the keys themselves to wrap them in single quotes
  which I expect to be clearer even if there are unfamiliar ones.

2. Storage Protocol compare method

As a secondary part of the same work, I also needed a compare method on StorageProtocol to make mine work, which was discussed here

We should also change how == works currently for external snapshots. It currently loads the value from disc and compares it. StorageProtocol needs an API to customize the comparison.

3. Don't increment missing_values in External base class __eq__ method if in create mode

• As in External base __eq__ causes test failure in teardown after new snapshot created #314

Nudge the increment line to after create so as not to break the test in teardown if it was just created

Related Issue(s)

• Support for perceptual hash snapshots with archived output files #311

Checklist

• I have tested my changes thoroughly (you can download the test coverage with hatch run cov:github).
  • I presume it won't but I have not tried setting up the test suite yet
• I have added/updated relevant documentation.
  • TODO
• [x] I have added tests for new functionality (if applicable). N/A
• I have reviewed my own code for errors.
  • Very minor change and checked it works with lmmx/inline-snapshot-phash
• I have added a changelog entry with hatch run changelog:entry
  • TODO
• I used semantic commits (feat: will cause a major and fix: a minor version bump)
  • TODO: amend commit to fix: and force push overwrite
• You can squash you commits, otherwise they will be squashed during merge

[lmmx]

Re-pushed to ensure tests pass incrementally

• All tests passing on new base commit ce43cde

[lmmx]

Re-pushed to reorder commits (protocol may need to be split off if it's the one causing test failures)

There are far fewer test failures on the (now 2nd) commit, the one that moves the missing values

It's possible these failures indicate this cannot be done though

FAILED tests/external/test_external.py::test_outsource - assert 0 == 1
FAILED tests/external/test_external.py::test_show_diff - assert 0 == 1
FAILED tests/external/test_external_file.py::test_external_in_other_dir - assert 0 == 1
FAILED tests/external/test_external_location.py::test_missing_suffix - assert 0 == 1
FAILED tests/external/test_formats.py::test_binary_format - assert 0 == 1
FAILED tests/external/test_formats.py::test_large_binary_format - assert 0 == 1
ERROR tests/external/storage/test_uuid.py::test_test_dir - Failed: some snapshots in this test have incorrect values.
ERROR tests/external/storage/test_uuid.py::test_trim - Failed: some snapshots in this test have incorrect values.
ERROR tests/external/test_external.py::test_tests_dir - Failed: some snapshots in this test have incorrect values.
ERROR tests/external/test_external_file.py::test_external_file - Failed: some snapshots in this test have incorrect values.
ERROR tests/external/test_formats.py::test_replace_format - Failed: some snapshots in this test have incorrect values.
ERROR tests/test_docs.py::test_docs[register_format.md] - Failed: some snapshots in this test have incorrect values.
ERROR tests/test_docs.py::test_docs[external.md] - Failed: some snapshots in this test have incorrect values.