This pre-release adds support for Protocol 18.

horizonclient

• The restriction that Fund can only be called on the DefaultTestNetClient has been removed. Any horizonclient.Client may now call Fund. Horizon instances not supporting funding will error with a resource not found error.
• Change AccountRequest to accept Sponsor and LiquidityPool filters
• Change EffectRequest, TransactionRequest, and OperationRequest to accept a ForLiquidityPool filter
• Change TradeRequest to accept both a ForLiquidityPool filter or a TradeType filter
• Add LiquidityPoolsRequest for getting details about liquidity pools, with an optional Reserves field to filter by pools' reserve asset(s).
• Add LiquidityPoolRequest for getting details about a specific liquidity pool via the LiquidityPoolID filter.

txnbuild

New features

• GenericTransaction, Transaction, and FeeBumpTransaction now implement
  encoding.TextMarshaler and encoding.TextUnmarshaler.
• New asset structures that conform to the new ChangeTrust and New assets:
• Support for the core liquidity pool XDR types: LiquidityPoolId, LiquidityPoolParameters, LiquidityPoolDeposit, and LiquidityPoolWithdraw.
• Support for the new asset structures: ChangeTrustAsset and TrustLineAsset.

Changes

• There's now a 5-minute grace period to transaction.ReadChallengeTx's minimum time bound constraint (#3824).
• Assets can now be liquidity pool shares (AssetTypePoolShare).
• All asset objects can now be converted to the new ChangeTrustAsset and TrustLineAsset objects.
• Assets can now be compared in accordance with the protocol, see their respective LessThan() implementations.

Breaking changes

• ChangeTrust requires a ChangeTrustAsset.
• RevokeSponsorship requires a TrustLineAsset when revoking trustlines.
• RemoveTrustlineOp helper now requires a ChangeTrustAsset
• validate*Asset helpers now require more-specific asset types.