[Enhancement] Invoke-GitHubAPI - Automatically set paging size

[MariusStorhaug]

Currently the paging size is set per command that supports it, but for simplicity this could be set in the Invoke-GHAPI function and overridden when wanted. i.e. the default value gets removed from the calling function, but remains a default in the Invoke-GHAPI function and overrideable from the calling function by providing a value to the pagesize param for the function.