Configurable relayers for "create account" flow

[DavidM-D]

When a SDK user creates account on their version of FastAuth we want them to be able to pay the Zero Balance Account creation gas fees.

Since we add each SDK user to the MPC service manually, we should also add their relayer of choice and create a way of authenticating the MPC service with the relayer for example by using a API key.

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1. Support multiple Firebase instances mpc-recovery#251
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[volovyks]

I have started implementing it, but basically it just a tiny part of #251
Or, at least we need to add support for multiple Firebase projects first.
cc @ChaoticTempest