Optimize data format key name regular expression

[ghost]

A regexploit analysis shows:

Vulnerable regex in ./intelmq/lib/message.py #115
Pattern: ^[a-z_](.[a-z_0-9]+)*$
Context: if not re.match('^[a-z_](.[a-z_0-9]+)*$', harm_key) and harm_key != '__type':
---
Redos(starriness=11, prefix_sequence=SEQ{ [5f:_,[a-z]] }, redos_sequence=SEQ{ [5f:_,[a-z],[0-9]]{2+}{0+} $[5f:_,[a-z],[0-9]] }, repeated_character=[5f:_,[a-z],[0-9]], killer=[^5f:_,[a-z],[0-9]])
Worst-case complexity: 11 ⭐⭐⭐⭐⭐⭐⭐⭐⭐⭐ (exponential)
Repeated character: [5f:_,[a-z],[0-9]]
Final character to cause backtracking: [^5f:_,[a-z],[0-9]]
Example: 'a' + '0' * 3456 + 'A'

The expression can be optimized and should be pre-compiled as well.

The check also only needs to be performed for the extra-keys, as the others are defined anyway.