Eliminate per-IAM-resource Merge functions

[melinath]

The Merge functions are always the same (and will always be the same for standard IAM resources). We should create a standard set of functions and eliminate the individual functions in each IAM resource conversion file.

# before
"google_bigquery_table_iam_binding": {
	{
		Convert:           GetBigQueryTableIamBindingCaiObject,
		MergeCreateUpdate: MergeBigQueryTableIamBinding,
		MergeDelete:       MergeBigQueryTableIamBindingDelete,
		Fetch:             FetchBigQueryTableIamPolicy,
	},
},

# after
"google_bigquery_table_iam_binding": {
	{
		Convert:           GetBigQueryTableIamBindingCaiObject,
		MergeCreateUpdate: MergeIamBinding,
		MergeDelete:       MergeIamBindingDelete,
		Fetch:             FetchBigQueryTableIamPolicy,
	},
},

For now we want to keep those fields in the mappers because we may need the flexibility in the future.

[marktru]

Internal issue: b/211902200