* Addresses clippy lints
* checks network magic and returns early from `is_regtest()`
* Moves `subsidy.rs` to `zebra-chain`, refactors funding streams into structs, splits them into pre/post NU6 funding streams, and adds them as a field on `testnet::Parameters`
* Replaces Vec with HashMap, adds `ConfiguredFundingStreams` type and conversion logic with constraints.
Minor refactors
* Empties recipients list
* Adds a comment on num_addresses calculation being invalid for configured Testnets, but that being okay since configured testnet parameters are checked when they're being built
* Documentation fixes, minor cleanup, renames a test, adds TODOs, and fixes test logic
* Removes unnecessary `ParameterSubsidy` impl for &Network, adds docs and TODOs
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Co-authored-by: Pili Guerra <mpguerra@users.noreply.github.com>
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