Add tool library for updating Spanner schema using Liquibase.

[SanjayVas]

This change is 

https://rally1.rallydev.com/#/?detail=/task/636975105303&fdp=true

[efoxepstein]

Reviewed 5 of 5 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @SanjayVas and @stevenwarejones)

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src/main/kotlin/org/wfanet/measurement/gcloud/spanner/tools/UpdateSchema.kt line 47 at r1 (raw file):

    val connectionString =
      if (flags.emulatorHost == null) {
        "jdbc:cloudspanner:/projects/${flags.projectName}/instances/${flags.instanceName}/" +

Maybe val databasePath = "projects/.../instances/.../databases/..." so you can reuse it between the two branches here (and also make it more compact).

That could even be a property on SpannerFlags.

[SanjayVas]

Reviewable status: 3 of 5 files reviewed, 1 unresolved discussion (waiting on @efoxepstein and @stevenwarejones)

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src/main/kotlin/org/wfanet/measurement/gcloud/spanner/tools/UpdateSchema.kt line 47 at r1 (raw file):

Previously, efoxepstein (Eli Fox-Epstein) wrote…

Maybe val databasePath = "projects/.../instances/.../databases/..." so you can reuse it between the two branches here (and also make it more compact).

That could even be a property on SpannerFlags.

Done, and moved to SpannerFlags.

[stevenwarejones]

Reviewed 3 of 5 files at r1, 2 of 2 files at r2, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @efoxepstein)

[stevenwarejones]

Reviewed 1 of 1 files at r3, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @efoxepstein)

[efoxepstein]

Reviewed 2 of 2 files at r2, 1 of 1 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @SanjayVas)