Sealevel balance metrics

[daniel-savu]

Publishes sealevel balance as a relayer metric

Drive-by Changes

• Creates HyperlaneSealevelError, a hyperlane-sealevel specific type to avoid including sealevel errors in hyperlane-core's ChainCommunicationError. It's pretty empty now, but we have a starting point now. This follows the approach used in hyperlane-cosmos too.
• Includes an rpc provider instance in SealevelProvider, and uses it to add get_balance on SealevelProvider.

Testing
None, as sealevel e2e is currently disabled

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Latest commit: db02400

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