Make it possible to ignore comment lines when comparing schemas

[jianglai]

We now pin to postgreSQL v17 when running tests, which means that minor
version might increase without our intervention. This causes (at least)
the comment in the golden schema to change, and failing the test as a
result.

This PR adds the ability to strip lines that we deem as comment from the
comparison, so we don't have to do trivial upgrades to the gold schema
whenever there's minor version upgrade.

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This change is 

[gbrodman]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @jianglai)

[CydeWeys]

Reviewable status: all files reviewed, 2 unresolved discussions

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common/src/testing/java/google/registry/testing/truth/TextDiffSubject.java line 89 at r1 (raw file):

  /** If set, ignore lines that start with the given string. */
  public TextDiffSubject ignoringLinesThatStartWith(String comment) {

I think it's cleaner if the method signature here is a varargs, and then comments can be of type ImmutableSet<String>. It'll be nicer from callsites as well, e.g. compare:

assertThat(...).ignoringLinesStartingWith("--", "//", "#").isEqualTo(...);

assertThat(...).ignoringLinesStartingWith("--").ignoringLinesStartingWith("//").ignoringLinesStartingWith("#").isEqualTo(...);

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common/src/testing/java/google/registry/testing/truth/TextDiffSubject.java line 89 at r1 (raw file):

  /** If set, ignore lines that start with the given string. */
  public TextDiffSubject ignoringLinesThatStartWith(String comment) {

It's minor but I think ignoringLinesStartingWith() might be a slightly better name.