EQL: Introduce explicit equality operator for strings

[costin]

Currently == has overloaded semantics namely:

• supporting case-insensitive comparison
• supporting "*" wildcard pattern.

which have proven problematic.
Instead of overloading ==, a new comparison operator is introduced that takes over these semantics, leaving == with exact ones.

The current proposals for the newly introduced operator are:
a. : / !:
b. =~ / !~

Regardless of the actual operator being used, the new operator will:

• always perform case-insensitive comparisons
• will support the "*" wildcard pattern, that is comparing it to a string literal that contains *, will be equivalent to a wildcard match. To escape * in a string, use \:

where a <op> `*`    // wildcard(a,"*")
where a <op> `foo*` // wildcard(a, "foo*")
where a <op> concat("foo", "*") // a == contact("foo", "*")
where a <op> `\*`   // a == "*"

• the operator does not combine with any other operator (>, >=, ==, etc..).
• there is no parameter, option or support for supporting the existing EQL behavior, that is == is always exact and there is no way to make == behave like the newly introduced operator.

Relates to (see #62651 (comment)).

[elasticmachine]

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

[costin]

Relates #62883

[costin]

Addressed through 201e577
Went ahead with : for operator. The negating operator !: hasn't not been added since most queries are about inclusion not exclusion.