tbv2: support capture-thrift-isset

[JakeHillion]

tbv2: support capture-thrift-isset

Support the capture-thrift-isset feature with TreeBuilder-v2. Fairly minor changes here except the type of the Enum in a template parameter now matters.

We follow the previous behaviour of capturing a value for each field in a struct that has an isset_bitset. This value is a VarInt captured before the C++ contents of the member. It has 3 values: 0 (not set), 1 (set), and 2 (unavailable). These are handled by the processor and represented in the output as false, true, and std::nullopt_t respectively.

Changes:

• Add a simple Thrift isset processor before any fields that have Thrift isset.
• Store the fully qualified names of enum types in DrgnParser - it already worked out this information anyway for naming the values and this is consistent with classes.
• Forward all enum template parameters under their input name under the assumption that they will all be policy type things like IssetBitsetOption. This could turn out to be wrong.

Test plan:

• CI (doesn't test thrift changes but covers other regressions)
• Updated Thrift enum tests for new format.
• FILTER='OilIntegration.*' make test - Thrift tests failed before, succeed after.

[codecov-commenter]

Codecov Report

Attention: 21 lines in your changes are missing coverage. Please review.

Comparison is base (4975b6e) 62.44% compared to head (351f2f8) 62.41%.

Files	Patch %	Lines
oi/CodeGen.cpp	54.28%	10 Missing and 6 partials :warning:
oi/type_graph/DrgnParser.cpp	75.00%	1 Missing and 3 partials :warning:
include/oi/exporters/Json.h	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #462 +/- ## ========================================== - Coverage 62.44% 62.41% -0.03% ========================================== Files 123 123 Lines 12021 12054 +33 Branches 1958 1965 +7 ========================================== + Hits 7507 7524 +17 - Misses 3569 3578 +9 - Partials 945 952 +7 ```

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