chore: Datadog PoC [OTE-702] [1/n]

[yogurtandjam]

Initial Datadog PoC according to tech spec

Followed by https://github.com/dydxprotocol/v4-web/pull/932/files

This PR:

• adds the base dd library and updates the analytics filepath to differentiate datadog and amplitude
• adds datadog context updating to the amplitude identity event, to make sure we get all the relevant user properties added automatically

This implementation is different from other tooling implementations (where they use an inject-x.js script)

Reasoning here: https://dydx-team.slack.com/archives/C042V24U87Q/p1724106274429279?thread_ts=1724101364.841789&cid=C042V24U87Q

[linear[bot]]

OTE-702 Datadog PoC

[vercel[bot]]

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