[C++][Python] Allow an ExtensionType to register or implement custom casts

With ARROW-14500 and ARROW-15545 (https://github.com/apache/arrow/pull/14106), we are allowing to cast "storage_type" -> "extension" (and the cast the other way around already worked as well).

Initially, that PR allowed any cast from "any" -> "extension", as long as the input type could be cast to the storage type (so deferring to the "any" -> "storage_type" cast). However, because whether a certain cast makes sense or not depends on the semantics of the extension type, it was restricted to exactly matching storage_type.

One idea could be to still allow the other casts behind a cast option flag, like `allow_non_storage_extension_casts` (or a better name), so the user can explicitly allow to cast to/from any type (as long as the cast from/to the storage type works).

That could help for the user, but for certain casts, the ExtensionType might also want to control _how_ such a cast is done. For example, for casting to/from string type (which would be useful for reading/writing CSV files, or for repr), you typically will want to do something different than casting your storage array to string.

A more general solution could thus be to have a mechanism for the ExtensionType to implement a certain cast kernel itself, and register this to the C++ cast dispatching.

**Reporter**: [Joris Van den Bossche](https://issues.apache.org/jira/browse/ARROW-17890) / @jorisvandenbossche

_{**Note**: *This issue was originally created as [ARROW-17890](https://issues.apache.org/jira/browse/ARROW-17890). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start by reading ARROW-14500, ARROW-15545, and pull request 14106 to understand the existing storage_type and extension casting behavior. Then compare the option-flag idea with the more general custom cast-kernel mechanism; done means an ExtensionType can control the relevant casts and the behavior is covered by the project's existing cast dispatching tests.

Written by the indexing model from the issue text.