Support Timestamp in proto to bigquery schema conversion

[Abacn]

For example, in PubSub_Proto_to_BigQuery_Xlang, when there is a Timestamp field, it get inferred to a BigQuery RECORD field of {"seconds": "INTEGER", "nanos": "INTEGER"}, i.e., google.protobuf.Timestamp gets expanded.

[Abacn]

Internal bug: 355704665

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 42.99%. Comparing base (9ee87ca) to head (102e966).
Report is 6 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1789      +/-   ##
============================================
- Coverage     43.02%   42.99%   -0.03%     
- Complexity     3404     3406       +2     
============================================
  Files           817      818       +1     
  Lines         47730    47783      +53     
  Branches       5122     5129       +7     
============================================
+ Hits          20535    20544       +9     
- Misses        25533    25578      +45     
+ Partials       1662     1661       -1     

Components	Coverage Δ
spanner-templates	63.88% <ø> (+0.01%)	⬆️
spanner-import-export	64.31% <ø> (+0.04%)	⬆️
spanner-live-forward-migration	75.17% <ø> (ø)
spanner-live-reverse-replication	71.62% <ø> (ø)
spanner-bulk-migration	83.85% <ø> (ø)

Files	Coverage Δ
...om/google/cloud/teleport/v2/utils/SchemaUtils.java	86.66% <100.00%> (+0.95%)	⬆️

... and 2 files with indirect coverage changes

[Abacn]

R: @Polber

[Abacn]

Four attempts all have different tests failing, not related to the change