gp init cluster: support for mirror functional tests

[srividyaky]

This commit adds functional tests for the changes committed in the gpinitsystem support for mirror PR https://github.com/greenplum-db/gpdb/pull/17294.

init_cluster_config_validation_test.go - This contains tests related to the input init config provided with support for mirrors and functions to form default config with primary and mirror support init_cluster_db_validation_test.go - This contains tests related to the database configuration checks after the cluster is successfully initialized with mirror support

Note: This PR cant be merged until https://github.com/greenplum-db/gpdb/pull/17294 is merged