feat: Add feature for copying backups

[danieljbruce]

This PR includes changes in the veneer layer that implement the new copy backup feature which allows users to copy a backup of a table to a cluster of their choice.

A config interface is introduced which is used to specify details about the copied backup. On a backup object, a copy function is added which copies the backup to a destination specified by the config passed into the function parameter. Code that parses expiry time is reused for the copy backup function.

Unit tests are added to make sure source and destination information get passed along to the Gapic layer. System tests are added to check that copying a backup works when copying to the same cluster, different cluster and different project. System tests are also added to be sure that expiry times can be passed in using different formats and tests are added so that we can confirm restoring the copied backup works properly.

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Warning: This pull request is touching the following templated files:

• .kokoro/system-test.sh - .kokoro files are templated and should be updated in synthtool

[danieljbruce]

https://github.com/googleapis/nodejs-bigtable/pull/1372 will address the failing test in the system tests.