Modify chain configuration in Bolt Boost

[thedevbirb]

Currently in the Bolt Boost configuration the chain field required in the TOML template we set either "Holesky" or "Mainnet" because that's what Commit Boost supports.

However, with latest releases (starting from v0.3.0) CB supports a Custom variant, see [https://github.com/Commit-Boost/commit-boost-client/pull/138]. After supporting that release on Bolt Boost as well we should probably change this value passed to the TOML template.