Add bidirectional conversion between scalar::BigInt and pb::eth::v2::BigInt

[codegen-sh]

This PR adds bidirectional conversion between scalar::BigInt and substreams_ethereum::pb::eth::v2::BigInt via the From trait.

Changes

• Added a new conversions.rs module with implementations of the From trait for both BigInt types
• Added tests for the conversions
• Added a new optional ethereum feature to enable Ethereum-specific functionality
• Updated the CHANGELOG.md to document the changes

Implementation Details

• The conversion from scalar::BigInt to pb::eth::v2::BigInt uses the to_signed_bytes_be method
• The conversion from pb::eth::v2::BigInt to scalar::BigInt uses the from_signed_bytes_be method
• The conversions are only available when the ethereum feature is enabled

Fixes BLO-318

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