deps: update core dependencies to v1.57.0 (#188)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [com.google.api:gax-grpc](https://togithub.com/googleapis/gax-java) | minor | `1.56.0` -> `1.57.0` |
| [com.google.api:gax-bom](https://togithub.com/googleapis/gax-java) | minor | `1.56.0` -> `1.57.0` |

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### Release Notes

<details>
<summary>googleapis/gax-java</summary>

### [`v1.57.0`](https://togithub.com/googleapis/gax-java/releases/v1.57.0)

[Compare Source](https://togithub.com/googleapis/gax-java/compare/v1.56.0...v1.57.0)

06-10-2020 15:01 PDT

##### Implementation Changes

-   logging: catch and log exceptions in Watchdog runnable to ensure it never stops running ([#&#8203;1036](https://togithub.com/googleapis/gax-java/pull/1036))
-   android: make GaxProperties.version accessible in Android ([#&#8203;1117](https://togithub.com/googleapis/gax-java/pull/1117))
-   Make watchdog `@BetaApi` and watchdog provider implementations `@InternalApi` ([#&#8203;1119](https://togithub.com/googleapis/gax-java/pull/1119))

##### New Features

-   generalize ExecutorService for transport layers ([#&#8203;1098](https://togithub.com/googleapis/gax-java/pull/1098))
-   add setAttemptDirectPath(boolean) for InstantiatingGrpcChannelProvider.Builder  ([#&#8203;1015](https://togithub.com/googleapis/gax-java/pull/1015))

##### Dependencies

-   update grpc to 1.30.0, protobuf to 3.12.2 ([#&#8203;1125](https://togithub.com/googleapis/gax-java/pull/1125))
-   update api-common-java to 1.92 ([#&#8203;1126](https://togithub.com/googleapis/gax-java/pull/1126))

##### Internal / Testing Changes

-   [gax-java] chore: include only external-facing gax testing utils in testlibJar ([#&#8203;1120](https://togithub.com/googleapis/gax-java/pull/1120))
-   ci: Linkage Checker to examine linkage error at artifact level ([#&#8203;1032](https://togithub.com/googleapis/gax-java/pull/1032))
-   ci: Adopt GitHub Actions in gax-java: unit tests and linkage monitor ([#&#8203;1062](https://togithub.com/googleapis/gax-java/pull/1062))
-   chore: display call settings configuration through toString ([#&#8203;1017](https://togithub.com/googleapis/gax-java/pull/1017))
-   test: cleaning up deprecated usages  ([#&#8203;858](https://togithub.com/googleapis/gax-java/pull/858))

</details>

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pom.xml

@@ -66,7 +66,7 @@
     <google.core.version>1.91.1</google.core.version>
     <google.api-common.version>1.9.2</google.api-common.version>
     <google.common-protos.version>1.18.0</google.common-protos.version>
-    <gax.version>1.56.0</gax.version>
+    <gax.version>1.57.0</gax.version>
     <grpc.version>1.30.0</grpc.version>
     <protobuf.version>3.12.2</protobuf.version>
     <junit.version>4.13</junit.version>