ESQL: Add case-insensitive behavior

[costin]

Description

Meta ticket for introducing case-insensitive string comparisons and functions:

• Introduce =~ operator for performing case insensitive equality between a string field and a string literal string expressions.
  The field can be both single and multi value. The literal has to be single value.
  When used on non-strings, the operator has == semantics
  The operator cannot be used on non-strings initially.
  ESQL: add =~ operator (case insensitive equality) #103656
• Introduce to_lower/lower_case and to_upper/upper_case
  ESQL: Add TO_UPPER and TO_LOWER functions #104309
• Potentially introduce like/rlike semantics to =~ depending on the string pattern

Postpone for now the items below:

• Introduce ~ suffix to make string operators and functions case insensitive
  • Extend grammar to allow ~ suffix for IN, LIKE, RLIKE operators
  • Extend grammar to allow ~ suffix for string functions
  • Extend existing string functions with case-insensitive behavior
  • Extend string filters to support case-insensitive behavior
  • Extend filters pushdown to allow case-insensitive

It's best to develop this feature on a branch against which PRs are created and once approved, merge the branch in main.

[elasticsearchmachine]

Pinging @elastic/es-analytics-geo (Team:Analytics)

[elasticsearchmachine]

Pinging @elastic/es-ql (Team:QL)