* Setting `script.painless.regex.enabled` has a new option,
  `use-factor`, the default.  This defaults to using regular
  expressions but limiting the complexity of the regular
  expressions.

  In addition to `use-factor`, the setting can be `true`, as
  before, which enables regular expressions without limiting them.

  `false` totally disables regular expressions, which was the
  old default.

* New setting `script.painless.regex.limit-factor`.  This limits
  regular expression complexity by limiting the number characters
  a regular expression can consider based on input length.

  The default is `6`, so a regular expression can consider
  `6` * input length number of characters.  With input
  `foobarbaz` (length `9`), for example, the regular expression
  can consider `54` (`6 * 9`) characters.

  This reduces the impact of exponential backtracking in Java's
  regular expression engine.

* add `@inject_constant` annotation to whitelist.

  This annotation signals that a compiler settings will
  be injected at the beginning of a whitelisted method.

  The format is `argnum=settingname`:
  `1=foo_setting 2=bar_setting`.

  Argument numbers must start at one and must be sequential.

* Augment
  `Pattern.split(CharSequence)`
  `Pattern.split(CharSequence, int)`,
  `Pattern.splitAsStream(CharSequence)`
  `Pattern.matcher(CharSequence)`
  to take the value of `script.painless.regex.limit-factor` as a
  an injected parameter, limiting as explained above when this
  setting is in use.

Fixes: elastic#49873