clients/horizonclient,txnbuild: update changelogs for v4.1.0 (#3136)

Update the changelogs for clients/horizonclient and txnbuild for the combined v4.1.0 release. Also add a missing empty release details to clients/horizonclient changelog that was missing in the previous release which only changed txnbuild.

There are several changes to txnbuild on master that could be released. I specifically want to get the SEP-10 v2.1.0 changes in a release.

There are only meaningful changes to txnbuild but txnbuild and horizonclient get released together, so that is why there are empty changelog release notes for the latter.

clients/horizonclient/CHANGELOG.md

@@ -3,6 +3,14 @@
 All notable changes to this project will be documented in this
 file.  This project adheres to [Semantic Versioning](http://semver.org/).
 
+## [v4.1.0](https://github.com/stellar/go/releases/tag/horizonclient-v4.1.0) - 2020-10-16
+
+None
+
+## [v4.0.1](https://github.com/stellar/go/releases/tag/horizonclient-v4.0.1) - 2020-10-02
+
+None
+
 ## [v4.0.0](https://github.com/stellar/go/releases/tag/horizonclient-v4.0.0) - 2020-09-29
 
 Added new client methods and effects supporting [Protocol 14](https://github.com/stellar/go/issues/3035).

txnbuild/CHANGELOG.md

@@ -3,7 +3,8 @@
 All notable changes to this project will be documented in this
 file.  This project adheres to [Semantic Versioning](http://semver.org/).
 
-## Unreleased
+## [v4.1.0](https://github.com/stellar/go/releases/tag/horizonclient-v4.1.0) - 2020-10-16
+
 * Add helper function `ParseAssetString()`, making it easier to build an `Asset` structure from a string in [canonical form](https://github.com/stellar/stellar-protocol/blob/master/ecosystem/sep-0011.md#asset) and check its various properties ([#3105](https://github.com/stellar/go/pull/3105)).
 
 * Add helper function `Transaction.ClaimableBalanceID()`, making it easier to calculate balance IDs for [claimable balances](https://developers.stellar.org/docs/glossary/claimable-balance/) without actually submitting the transaction ([#3122](https://github.com/stellar/go/pull/3122)).