Make DEFAULT_LOCATOR Configurable

[heka1024]

In my application, I want all select query from GlobalID::Locator to come from the read only database, but I can't modify the DEFAULT_LOCATOR, so I made it configurable. I think it's more in line with the Rails philosophy to make this customizable, not just for my use case.

Any feedback or suggestions would be greatly appreciated!

[heka1024]

Because every version of railties depends on Ruby >= 3.1.0
  and rails_main.gemfile depends on railties >= 0,
  Ruby >= 3.1.0 is required.
So, because current Ruby version is = 2.7.8,
  version solving has failed.

There are some unrelated CI issues that should be handled in a separate PR. I'll take care of them when I have time.

[heka1024]

Hi, @rafaelfranca. Could you give me a feedback when you have a time?

[olleolleolle]

@heka1024 I think you can rebase these changes, and tests may all pass?

[heka1024]

@olleolleolle All tests are passing now. Appreciate the help!