Merge bitcoin/bitcoin#25001: Modernize util/strencodings and util/string: `string_view` and `optional`

fa7078d84fc2858a466bc1a85404f821df682538 scripted-diff: Rename ValidAsCString to ContainsNoNUL (MacroFake)
e7d2fbda63c346ae88767c3f8d4db3edeae2dc0b Use std::string_view throughout util strencodings/string (Pieter Wuille)
8ffbd1412d887535ce5eb613884858c319bd12be Make DecodeBase{32,64} take string_view arguments (Pieter Wuille)
1a72d62152bfdd7c5c2b2704b679f894e7d35e37 Generalize ConvertBits to permit transforming the input (Pieter Wuille)
78f3ac51b7d073d12da6a3b9b7d80d91e04ce3a7 Make DecodeBase{32,64} return optional instead of taking bool* (Pieter Wuille)
a65931e3ce66d87b8f83d67ecdbb46f137e6a670 Make DecodeBase{32,64} always return vector, not string (Pieter Wuille)
a4377a0843636eae0aaf698510fc6518582545db Reject incorrect base64 in HTTP auth (Pieter Wuille)
d648b5120b2fefa9e599898bd26f05ecf4428fac Make SanitizeString use string_view (Pieter Wuille)
963bc9b576f0a62caffede2ce32830aef3473995 Make IsHexNumber use string_view (Pieter Wuille)
40062997f223d88d4f92aaae4622a31476686163 Make IsHex use string_view (Pieter Wuille)
c1d165a8c2678c31aced5e1d46231d9996b0774a Make ParseHex use string_view (Pieter Wuille)

Pull request description:

  Make use of `std::string_view` and `std::optional` in the util/{strencodings, string} files.

  This avoids many temporary string/vector objects being created, while making the interface easier to read. Changes include:
  * Make all input arguments in functions in util/strencodings and util/string take `std::string_view` instead of `std::string`.
  * Add `RemovePrefixView` and `TrimStringView` which also *return* `std::string_view` objects (the corresponding `RemovePrefix` and `TrimString` keep returning an `std::string`, as that's needed in many call sites still).
  * Stop returning `std::string` from `DecodeBase32` and `DecodeBase64`, but return vectors. Base32/64 are fundamentally algorithms for encoding bytes as strings; returning `std::string` from those (especially doing it conditionally based on the input arguments/types) is just bizarre.
  * Stop taking a `bool* pf_invalid` output argument pointer in `DecodeBase32` and `DecodeBase64`; return an `std::optional` instead.
  * Make `DecodeBase32` and `DecodeBase64` more efficient by doing the conversion from characters to integer symbols on-the-fly rather than through a temporary vector.

ACKs for top commit:
  MarcoFalke:
    re-ACK fa7078d84fc2858a466bc1a85404f821df682538 only change is rebase and adding a scripted-diff 🍲
  martinus:
    Code review ACK fa7078d84fc2858a466bc1a85404f821df682538, found no issue
  laanwj:
    Code review ACK  fa7078d84fc2858a466bc1a85404f821df682538
  sipa:
    utACK fa7078d84fc2858a466bc1a85404f821df682538 (as far as the commit that isn't mine goes)

Tree-SHA512: 5cf02e541caef0bcd100466747664bdb828a68a05dae568cbcd0632a53dd3a4c4e85cd8c48ebbd168d4247d5c9666689c16005f1c8ad75b0f057d8683931f664

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