Add Regtest network

[alexeykoren]

User description

Add Regtest network

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PR Type

enhancement

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Description

• Introduced a new RegtestNetwork struct to support the regtest network.
• Implemented the Parameters trait for RegtestNetwork, defining its network type and activation heights.
• Added RegtestNetwork to the Network enum and updated the Parameters implementation to handle it.
• This enhancement allows the system to recognize and operate with the regtest network configuration.

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Changes walkthrough 📝

	Relevant files
Enhancement	consensus.rs Add support for Regtest network in consensus module            components/zcash_protocol/src/consensus.rs Added a new RegtestNetwork struct and implemented the Parameters trait for it. Introduced a constant REGTEST_NETWORK for the regtest network. Updated the Network enum to include RegtestNetwork. Modified the Parameters implementation for Network to handle RegtestNetwork. +36/-2

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[PaulLaux]

approved with one comment

[PaulLaux]

should probably keep hash here and in pub struct RegtestNetwork;