[DocDB] Add capability to add flag validator that can compare multiple gFlags values

[yugabyte-ci]

Jira Link: DB-11866

Some flag validation may depend on the value of another flag. Flags are parsed, validated and set in order they are passed in via the command line, or flags file. In order to not impose a restriction on the user to pass the flags in a particular obscure add a macro that delays the validation until all flags have been set.