fix: for Unable to configure Custom Domain for React-Native

[sunitaprajapati89]

fix: for Unable to configure Custom Domain for React-Native

[wenxi-zeng]

unblock e2e tests

the following should unblock the e2e the tests:

1. update the url in this block to match the changes you made (i.e. /events to v1/b, /settings to /projects/${writekey}/settings).
2. do the same for this block

NOTE that you need provide the write key to the endpoints

best practice

however, your changes hardcoded the path/endpoints to every domain, which disables the user to fully customize the url to their own endpoints and this is a breaking change. the suggested fix would be:

1. add a new flag in config, something like useSegmentEndpoints and default it to false. (so we don't break existing customer who use their own path/endpoints`
2. update your code to append Segment's path/endpoints if and only if useSegmentEndpoints is set to true. otherwise, use the one user provided as is
3. now you can revert the changes you made on the mock server (mentioned in unblock e2e tests section)
4. add unit tests to test against useSegmentEndpoints
5. update readme to reflect this change

[wenxi-zeng]

LGTM. just address the only comment and update README and it's ready to go