Firestore:Fix make change in CustomClassMapper serialize and deserialize methods to public

[abhinav-qlogic]

Fixes#5809

[dmitry-s]

Hi
just wanted to point out that the class is currently package-private. It needs to be public so we can use it outside of the package. Otherwise looks good!

[codecov]

Codecov Report

Merging #5879 into master will decrease coverage by 0.28%.
The diff coverage is 0%.

@@ Coverage Diff @@## master #5879 +/- ##
============================================
- Coverage 46.79% 46.51% -0.29% 
Complexity 25648 25648 ============================================
Files 2456 2456 Lines 267593 267477 -116 Branches 30564 29752 -812 ============================================
- Hits 125229 124408 -821 + Misses 133105 133026 -79 - Partials 9259 10043 +784

Impacted Files	Coverage Δ	Complexity Δ
.../com/google/cloud/firestore/CustomClassMapper.java	77.29% <0%> (ø)	102 <0> (ø)	⬇️
...va/com/google/cloud/compute/v1/InstanceClient.java	48.8% <0%> (-6.45%)	143% <0%> (ø)
.../com/google/cloud/compute/v1/RegionDiskClient.java	47.9% <0%> (-5.99%)	43% <0%> (ø)
...ava/com/google/cloud/compute/v1/ProjectClient.java	51.14% <0%> (-5.94%)	55% <0%> (ø)
...ava/com/google/cloud/compute/v1/NetworkClient.java	49.63% <0%> (-5.84%)	35% <0%> (ø)
...ava/com/google/cloud/compute/v1/LicenseClient.java	50.41% <0%> (-5.79%)	31% <0%> (ø)
...va/com/google/cloud/compute/v1/SnapshotClient.java	50% <0%> (-5.74%)	31% <0%> (ø)
.../java/com/google/cloud/compute/v1/ImageClient.java	49.04% <0%> (-5.74%)	43% <0%> (ø)
.../google/cloud/compute/v1/TargetSslProxyClient.java	48.57% <0%> (-5.72%)	35% <0%> (ø)
.../google/cloud/compute/v1/SecurityPolicyClient.java	48.1% <0%> (-5.7%)	39% <0%> (ø)
... and 257 more

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[BenWhitehead]

Superseded by #6085