Improve return typing on icache getOrSet

[KaneFreeman]

Type: feature

The following has been addressed in the PR:

• There is a related issue
• All code has been formatted with prettier as per the readme code style guidelines
• Unit or Functional tests are included in the PR

Description:

Improve argument and return typing on icache middleware's getOrSet method.

This is completely a typing change with no implementation changes.

Before:

icache.getOrSet('test', 'test'); /* Type: any */
icache.getOrSet('test', () => 'test'); /* Type: any */
icache.getOrSet('test', () => Promise.resolve()); /* Type: any */
	
icache.getOrSet<string>('test', 'test'); /* Type: string | undefined */
icache.getOrSet<string>('test', 42); /* No Error, Type: string | undefined */
icache.getOrSet<string>('test', () => 'test'); /* Type: string | undefined */
icache.getOrSet<string>('test', () => Promise.resolve()); /* Type: string | undefined */

After:

icache.getOrSet('test', 'test'); /* Type: 'test' */
icache.getOrSet('test', () => 'test'); /* Type: string */
icache.getOrSet('test', () => Promise.resolve()); /* Type: undefined */

icache.getOrSet<string>('test', 'test'); /* Type: string */
icache.getOrSet<string>('test', 42); /* Type error: '42' is not assignable to parameter of type 'string' */
icache.getOrSet<string>('test', () => 'test'); /* Type: string */
icache.getOrSet<string>('test', () => Promise.resolve()); /* Type error: '() => Promise<void>' is not assignable to parameter of type 'string' */
icache.getOrSet<string>('test', () => new Promise<string>(() => {})); /* Type: undefined */

[matt-gadd]

@KaneFreeman nice! 👍