fix(cdk/coercion): add the support for readonly array coercion

Fixes a type definition of coerceArray so that it accepts both mutable and
immutable arrays.

Fixes #18806

Author: klemenoslaj

src/cdk/coercion/array.ts

@@ -7,6 +7,6 @@
  */
 
 /** Wraps the provided value in an array, unless the provided value is an array. */
-export function coerceArray<T>(value: T | T[]): T[] {
+export function coerceArray<T>(value: T | readonly T[]): T[] {
   return Array.isArray(value) ? value : [value];
 }