Merge #18034: Get the OutputType for a descriptor

7e80f646b24a2abf3c031a649bcc706a695f80da Get the OutputType for a descriptor (Andrew Chow)

Pull request description:

  Adds a `GetOutputType()` method to get the OutputType of a descriptor. Some descriptors don't have a determinate OutputType, so we actually use an `Optional<OutputType>`. For descriptors with indeterminate OutputType, we return `nullopt`.

  `addr()` and `raw()` use OutputTypes as determined by the CTxDestination they have. For simplicity, `ScriptHash` destinations are `LEGACY` even though they could be `P2SH_SEGWIT`.
  `combo()`, `pk()`, and `multi()` are `nullopt` as they either don't have an OutputType or they have multiple. `DescriptorImpl` defaults to `nullopt`.
  `pkh()` is `LEGACY` as expected
  `wpkh()` and `wsh()` are `BECH32` as expected.
  `sh()` checks whether the sub-descriptor is `BECH32`. If so, it is `P2SH_SEGWIT`. Otherwise it is `LEGACY`.

  The descriptor tests are updated to check the OutputType too.

ACKs for top commit:
  fjahr:
    ACK 7e80f646b24a2abf3c031a649bcc706a695f80da
  meshcollider:
    utACK 7e80f646b24a2abf3c031a649bcc706a695f80da
  instagibbs:
    cursory ACK https://github.com/bitcoin/bitcoin/pull/18034/commits/7e80f646b24a2abf3c031a649bcc706a695f80da
  Sjors:
    Code review ACK 7e80f646b24a2abf3c031a649bcc706a695f80da
  jonatack:
    ACK 7e80f64 code review/build/tests

Tree-SHA512: c5a813447b62e982435e1c948066f8d6c148c9ebffb0a5eb5a9028b173b01d5ead2f076a5ca3f7f37698538baa346f82a977ee48f583d89cb4e5ebd9111b2341

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