feat: allow restore backup to different instance

[liubonan]

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Fixes #789
kokoro:force-run

[codecov]

Codecov Report

Merging #515 (b168074) into master (b4dfddd) will increase coverage by 0.02%.
The diff coverage is 88.23%.

@@             Coverage Diff              @@
##             master     #515      +/-   ##
============================================
+ Coverage     83.44%   83.47%   +0.02%     
- Complexity     1311     1314       +3     
============================================
  Files           114      114              
  Lines          7799     7806       +7     
  Branches        442      446       +4     
============================================
+ Hits           6508     6516       +8     
+ Misses         1036     1034       -2     
- Partials        255      256       +1     

Impacted Files	Coverage Δ	Complexity Δ
.../bigtable/admin/v2/models/RestoreTableRequest.java	82.85% <88.23%> (+4.28%)	10.00 <3.00> (+3.00)
...om/google/cloud/bigtable/emulator/v2/Emulator.java	60.65% <0.00%> (+0.81%)	14.00% <0.00%> (ø%)

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[igorbernstein2]

prod code looks good, but as @kolea2 pointed out, the integration tests need to be rebased

[generated-files-bot]

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

• .github/generated-files-bot.yml
• .kokoro/release/publish_javadoc.sh
• samples/install-without-bom/pom.xml
• samples/snapshot/pom.xml
• samples/snippets/pom.xml

[snippet-bot]

Here is the summary of changes.

You added 1 region tag.

• samples/snippets/src/main/java/com/m/examples/bigtable/HelloWorld.java:168, tag bigtable_hw_get_by_key_veneer

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[google-cla]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
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[kolea2]

@googlebot

[igorbernstein2]

lgtm