EQL: implement number function

[rw-access]

Part of #51556
https://eql.readthedocs.io/en/latest/query-guide/functions.html#number
number(input: str, [base: int]) -> int|float

Function behavior

Validation requirements:

• Accepts isString()
• Return null if input is null
• Autodetect the base if none provided
  • input startswith "0x": 16
  • otherwise: 10
• Tries to parse as integer. If that succeeds, it's returned as an integer
• If non-integer, then try to parse as a float. If that doesn't succeed, raise an EQL illegal argument exception. or is it better to raise the original exception?

[elasticmachine]

Pinging @elastic/es-ql (:Query Languages/EQL)

[rw-access]

7.x backport 61e2cf8