Implement hashlib.new('sha256') and enable it for CLUE

[samblenny]

The point of this is to:

1. Make it possible to use a CLUE board as a TOTP token that supports the SHA1 and SHA256 hash algorithm options for fast updating (pure python hashes take 2+ seconds)
2. Add optional SHA256 support to hashlib that can be used by other things that need it (e.g. CircuitMatter)

List of specific changes:

1. Implement 'sha256' option for hashlib.new()
2. Gate SHA256 stuff behind #ifdef CIRCUITPY_HASHLIB_SHA256
3. Enable hashlib with 'sha1' and 'sha256' for ports/nordic/clue_nrf52840_express

Checklist:

• pre-commit passed
• builds without errors
• test code runs on CLUE board with same results as CPython

Test Code:

Adafruit CircuitPython 10.1.0-beta.1-5-g108860a837-dirty on 2025-12-05; Adafruit CLUE nRF52840 Express with nRF52840
>>> import hashlib
>>> s1 = hashlib.new('sha1', b'SHA1 Hash')
>>> s2 = hashlib.new('sha256', b'SHA256 Hash')
>>> s1.digest().hex()
'a28191ff62cce66838e67c16dd48ab118c27b417'
>>> s2.digest().hex()
'2023b38445ae586b672848c31e3e13883f17fd07531c9aa2b8f212b3b7a0d11f'