The function `ExtractPubKey()` checks if a given script matches the P2PK pattern
(`<PubKey> OP_CHECKSIG`), extracts the PubKey and additionally checks if it is
cryptographically valid (full validation with ECC library via `CPubKey::IsFullyValid()`).

Currently this is done manually in the following order:
1. check if first script OP is data push with valid PubKey length (first part of pattern match), extract PubKey
2. create `CPubKey` object with extracted PubKey
3. fully validate public key
4. check if last script OP is `OP_CHECKSIG` (second part of pattern match)

Using Solver, the pattern matching and PubKey extraction can be done via a
single step, leading to the following simplified order with shorter code:
1. check if given script matches P2PK pattern with Solver (also contains valid PubKey length check), extracts Pubkey
2. create `CPubKey` object with extracted Pubkey
3. fully validate public key

ninja check check-functional