GH-43787: [C++] Register the new Opaque extension type by default

[rok]

This is to resolve #43787

The Opaque extension type implementation for C++ (plus python bindings) was added in #43458, but it was not registered by default, which we should do for canonical extension types (see #43458 (comment))

Additionally, this adds bool8 extension type builds with ARROW_JSON=false as discussed here

Rationale for this change

Canonical types should be registered by default if possible (except e.g. if they can't be compiled due to ARROW_JSON=false).

What changes are included in this PR?

This adds default registration for opaque, changes when bool8 is built and moves all canonical tests under the same test target.

Are these changes tested?

Changes are tested by previously existing tests.

Are there any user-facing changes?

opaue will now be registered by default and bool8 will be present in case ARROW_JSON=false at build time.

• GitHub Issue: [C++] Register the new Opaque extension type by default #43787

[rok]

Fixed a minor nit in the cpp/src/arrow/extension_type.cc, will merge if CI passes.

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 4 benchmarking runs that have been run so far on merge-commit b4f7efe.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 11 possible false positives for unstable benchmarks that are known to sometimes produce them.