feat(signing-fireblocks): make coin type configurable with backward-c…

[HelmiTrabelsi]

Motivation

CC_COIN_TYPE was hardcoded to 6767 inside FireblocksHandler, making it
impossible to configure a different derivation path coin type without forking
the package. This change makes it configurable while keeping full backward
compatibility.

Changes

• Add optional coinType?: number to FireblocksConfig and FireblocksHandler
  constructor. It defaults to CC_COIN_TYPE (6767) when omitted
• CC_COIN_TYPE = 6767 remains exported from core-signing-lib unchanged
• All existing call sites work without any modification

Usage

// existing — no change needed, still uses 6767
new FireblocksSigningDriver({ userApiKeys })

// new — opt in to a custom coin type
new FireblocksSigningDriver({ userApiKeys, coinType: 1234 })