feat(integration): unify base URL config and add region dropdown with manual override

[afsuyadi]

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Changes

Contributes to #7795

1. Add options array with label and value (URL) key for Amplitude, Datadog, Heap, Mixpanel, and New Relic integration
2. Modify CreateEditIntegration component to enable Custom URL as manual override

How did you test this code?

For every platform, I try to manually test them by choosing the options in Select component, either selecting the existing options or manually inserting the URL

Datadog testing:
datadog-testing.webm

Amplitude testing:
amplitude-testing.webm

Heap testing:
heap-testing.webm

New Relic testing:
new-relic_testing.webm

Mixpanel testing:
mixpanel-testing.webm

Additional Note

1. During Mixpanel's testing, it seems that the backend serializer haven't existed yet. Mentioning this unmerged PR fix: add regional base URL for Mixpanel #7802 which is related to the serializer of Mixpanel.
2. I plan to also add dropdown for LaunchDarkly, but it seems that it requires backend changes (new model field, migration, serializer update, client update), which seems to be out of this issue's scope. Waiting for further instructions.

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