fix(forge): custom verifiers should not throw on unknown etherscan chains

[grandizzy]

Motivation

• closes bug(forge verify-contract): Etherscan API URL Error when using blockscout #11430
• config should not throw on non etherscan verifiers when chain not known by etherscan - issue introduced in https://github.com/foundry-rs/foundry/pull/11194/files#diff-0f348074359029c2930eb72b52f482c2e0f4495dc61b95bc919f155a8d2fd82cR1437-R1444. Instead should just continue as unknown etherscan chains could be supported by other verifiers like blockscout and sourcify.

Solution

• in place fix, needs a broader refactoring to reflect same code is used by etherscan-like verifiers and more tests for non-etherscan supported verifiers
• consider backport to v1.3.3

PR Checklist

• Added Tests
• Added Documentation
• Breaking changes