Add SimpleGraph; minimal graph needed to read experimental tests

[jpschorr]

• Adds reader for 'experimental' graph conformance test data.
• Adds minimal SimpleGraph to hold the decoded data.
• No operations on the graph are implemented as yet; this is to unblock running the conformance tests.

To run the tests:

• cargo test --package partiql-conformance-tests --features "conformance_test, experimental" --release partiql_tests::eval::experimental::graph
• These will not run in github actions, as 'experimental' tests are disabled by default

Before this change:

failures:

---- partiql_tests::eval::experimental::graph::graph::directionality::directionality::permissive_left_undirected_with_variables_and_label stdout ----

thread 'partiql_tests::eval::experimental::graph::graph::directionality::directionality::permissive_left_undirected_with_variables_and_label' panicked at partiql-conformance-tests/tests/test_value.rs:54:10:
value decode to succeed: UnsupportedType("sexp")

... <snip> ...

test result: FAILED. 0 passed; 170 failed; 0 ignored; 0 measured; 6429 filtered out; finished in 0.01s

After this change:

failures:

---- partiql_tests::eval::experimental::graph::graph::directionality::directionality::permissive_left_right_with_variables_and_label stdout ----

thread 'partiql_tests::eval::experimental::graph::graph::directionality::directionality::permissive_left_right_with_variables_and_label' panicked at partiql-conformance-tests/tests/partiql_tests/eval/experimental/graph.rs:376:17:
When evaluating (mode = Coerce) `(pairs MATCH (x:a1)<-[y]->(z))`, unexpected parse error

... <snip> ...

test result: FAILED. 0 passed; 170 failed; 0 ignored; 0 measured; 6429 filtered out; finished in 0.02s

All the same tests still fail, but they fail on parsing the query, as GPML parsing/ast is not yet implemented. Previously, the test failed even before the query could be parsed, as the test itself could not load the test data.

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[github-actions]

Conformance comparison report

	Base (e47058e)	b4fb2a2	+/-
% Passing	87.15%	87.15%	0.00%
✅ Passing	5603	5603	0
❌ Failing	826	826	0
🔶 Ignored	0	0	0
Total Tests	6429	6429	0

Number passing in both: 5603

Number failing in both: 826

Number passing in Base (e47058e) but now fail: 0

Number failing in Base (e47058e) but now pass: 0

[codecov]

Codecov Report

Attention: Patch coverage is 72.10031% with 89 lines in your changes missing coverage. Please review.

Project coverage is 81.23%. Comparing base (e47058e) to head (b33bffd).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
extension/partiql-extension-ion/src/decode.rs	66.87%	10 Missing and 44 partials ⚠️
partiql-value/src/graph.rs	85.60%	19 Missing ⚠️
partiql-value/src/pretty.rs	11.11%	8 Missing ⚠️
partiql-value/src/value.rs	66.66%	3 Missing ⚠️
extension/partiql-extension-ion/src/boxed_ion.rs	0.00%	2 Missing ⚠️
partiql-logical/src/util.rs	0.00%	2 Missing ⚠️
extension/partiql-extension-ion/src/encode.rs	0.00%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (72.10%) is below the target coverage (85.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #546      +/-   ##
==========================================
- Coverage   81.37%   81.23%   -0.14%     
==========================================
  Files          98       99       +1     
  Lines       20870    21178     +308     
  Branches    20870    21178     +308     
==========================================
+ Hits        16982    17205     +223     
- Misses       3421     3462      +41     
- Partials      467      511      +44     

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