GH-45798: [C++] Add extension directory to Meson

[WillAyd]

Rationale for this change

Continues growing Meson support in Arrow

What changes are included in this PR?

Added the extension directory to the Meson configuration

Are these changes tested?

Yes

Are there any user-facing changes?

No

• GitHub Issue: [C++] Add extension directory to Meson #45798

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[WillAyd]

This all compiles fine, although at runtime I am getting a few test failures:

[ RUN      ] TestExtensionType.RoundtripBatch
../src/arrow/testing/gtest_util.cc:560: Failure
Failed
Left schema: f0: extension<arrow.fixed_shape_tensor[value_type=int64, shape=[3,4], dim_names=[x,y]]>
Right schema: f0: fixed_size_list<item: int64>[12]
-- metadata --
ARROW:extension:metadata: {"shape":[3,4],"dim_names":["x","y"]}
ARROW:extension:name: arrow.fixed_shape_tensor

../src/arrow/testing/gtest_util.cc:560: Failure
Failed
Left schema: f0: extension<arrow.fixed_shape_tensor[value_type=int64, shape=[3,4], dim_names=[x,y]]>
Right schema: f0: fixed_size_list<item: int64>[12]
-- metadata --
ARROW:extension:metadata: {"shape":[3,4],"dim_names":["x","y"]}
ARROW:extension:name: arrow.fixed_shape_tensor

...

[ RUN      ] TestExtensionType.RoundtripBatchFromTensor
../src/arrow/testing/gtest_util.cc:560: Failure
Failed
Left schema: f0: extension<arrow.fixed_shape_tensor[value_type=int64, shape=[3,4], dim_names=[x,y]]>
-- metadata --
ARROW:extension:metadata: {"shape":[3,4],"dim_names":["x","y"]}
ARROW:extension:name: arrow.fixed_shape_tensor
Right schema: f0: fixed_size_list<item: int64>[12]
-- metadata --
ARROW:extension:metadata: {"shape":[3,4],"dim_names":["x","y"]}
ARROW:extension:name: arrow.fixed_shape_tensor
ARROW:extension:metadata: {"shape":[3,4],"dim_names":["x","y"]}
ARROW:extension:name: arrow.fixed_shape_tensor

...

[ RUN      ] OpaqueType.BatchRoundTrip
../src/arrow/extension/opaque_test.cc:191: Failure
Expected equality of these values:
  *batch->schema()
    Which is: field: extension<arrow.opaque[storage_type=binary, type_name=geometry, vendor_name=adbc.postgresql]>
  *written->schema()
    Which is: field: binary

@lidavidm I see you are the only one who has touched the opaque file. Any idea what I should be looking at to debug the opaque test failure?