feat: add a two-arg variant of substring (#513)

Co-authored-by: Gil Forsyth <gforsyth@users.noreply.github.com>

extensions/functions_string.yaml

@@ -50,7 +50,9 @@ scalar_functions:
  name: substring
  description: >-
  Extract a substring of a specified `length` starting from position `start`.
- A `start` value of 1 refers to the first characters of the string.
+ A `start` value of 1 refers to the first characters of the string. When
+ `length` is not specified the function will extract a substring starting
+ from position `start` and ending at the end of the string.
  impls:
  - args:
  - value: "varchar<L1>"
@@ -76,6 +78,24 @@ scalar_functions:
  - value: i32
  name: "length"
  return: "string"
+ - args:
+ - value: "varchar<L1>"
+ name: "input"
+ - value: i32
+ name: "start"
+ return: "varchar<L1>"
+ - args:
+ - value: "string"
+ name: "input"
+ - value: i32
+ name: "start"
+ return: "string"
+ - args:
+ - value: "fixedchar<l1>"
+ name: "input"
+ - value: i32
+ name: "start"
+ return: "string"
  -
  name: regexp_match_substring
  description: >-