feat: adding ability to specify firewall network

[apankratov]

Fixes #125

• Tests and linter pass

• Code coverage does not decrease (if any source code was changed)

• Appropriate docs were updated (if necessary)

• added possibility to specify non-default network for setMetadata firewall method

[stephenplusplus]

Thank you, it all looks good to me!

Regarding the $MATCH pattern, that change would have to be made upstream in a generator library we use. Let's not hang this PR on it, but @JustinBeckwith-- do we want something like that?

[stephenplusplus]

Also, could you run npm run prettier? That should auto-correct the linting failures: https://circleci.com/gh/googleapis/nodejs-compute/4000

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@45aa29c). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff           @@
##             master   #150   +/-   ##
=======================================
  Coverage          ?   100%           
=======================================
  Files             ?     19           
  Lines             ?   1581           
  Branches          ?      0           
=======================================
  Hits              ?   1581           
  Misses            ?      0           
  Partials          ?      0

Impacted Files	Coverage Δ
src/firewall.js	100% <100%> (ø)

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[stephenplusplus]

@JustinBeckwith cla help, please!

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