add cache to get_schema, combine instance checks with validate

[braingram]

Here are a few optimizations.

1. Cache the retrieved_schema (as you suggested)
2. Combine the additional_validators with the new _Validate.iter_errors.

Combining the additional_validators with _Validate.iter_errors removes 1 walk of the tree during validation.

However, these changes don't appear to drastically speed up the test suite (it's still 63-65 seconds on my computer). This is possibly because the test files/trees are mostly small. For a random jwst asdf file I found in my downloads (jwst_nirspec_ifupost_0012.asdf) validation takes 964 ms without the changes and 599 ms with the changes in this PR.

The following tests run slowly with jsonschema 4.18:

• test_load_schema (and other test_load_schema_* tests): For example, test_load_schema_with_full_tag takes ~2 seconds with 4.17 and ~3 seconds with 4.18. The difference is mostly in the call to validate in asdf.schema.check_schema and appears to be attributable to jsonschema and not to anything we're doing.
• test_large_block_index: ~3 seconds with 4.17, ~4.5 seconds with 4.18. Looking at a profile of the test, in 4.17 ~1 second is spend in descend (in jsonschema, not the one in asdf.schema) whereas in 4.18 descend takes up ~2.8 seconds. line_profiler doesn't seem to play nice with the dynamic class generation and isn't providing meaningful results when I try to profile lines in descend.

I think this is far as I'm going to get on this today but I'll update if I get more time to look at it (and find anything useful).

[codecov-commenter]

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[eslavich]

LGTM!