Descriptor parser incorrectly accepts whitespace within key

[brunoerg]

The descriptor parser is currently accepting descriptors that contain whitespace within key (I did not check if it's accepting whitespace for other fragments).

Example

The following descriptor is incorrectly accepted:

tr(0000000000000000000000000000000000000000000009223372036854 775807)

This descriptor contains a space character within the 32-byte hex-encoded public key, which should cause parsing to fail. The parser should reject this descriptor with an error indicating invalid key format or invalid hex encoding, since: 1. The x-only public key should be exactly 64 contiguous hex characters. 2. Allowing whitespace breaks descriptor canonicalization.

[brunoerg]

For reference: bitcoinfuzz/bitcoinfuzz#292