feat: transaction callable as functional interface

[thiagotnunes]

Makes the TransactionCallable a @FunctionalInterface so it can be used with lambdas.

This PR also modifies all of its usages to the lambda syntax. Only syntatic changes were made, no logic changes were made here.

[codecov]

Codecov Report

Merging #1066 (52763a4) into master (1d4eed4) will increase coverage by 0.00%.
The diff coverage is 85.71%.

@@            Coverage Diff            @@
##             master    #1066   +/-   ##
=========================================
  Coverage     85.24%   85.25%           
- Complexity     2658     2660    +2     
=========================================
  Files           155      155           
  Lines         14453    14448    -5     
  Branches       1362     1362           
=========================================
- Hits          12320    12317    -3     
+ Misses         1565     1562    -3     
- Partials        568      569    +1     

Impacted Files	Coverage Δ	Complexity Δ
...java/com/google/cloud/spanner/AsyncRunnerImpl.java	93.75% <60.00%> (-0.19%)	10.00 <1.00> (+1.00)	⬇️
...cloud/spanner/connection/SingleUseTransaction.java	91.19% <92.85%> (-0.14%)	36.00 <0.00> (ø)
...ain/java/com/google/cloud/spanner/SessionImpl.java	87.13% <100.00%> (-0.08%)	35.00 <1.00> (+1.00)	⬇️
...a/com/google/cloud/spanner/SessionPoolOptions.java	69.53% <0.00%> (ø)	18.00% <0.00%> (+1.00%)
...ain/java/com/google/cloud/spanner/SessionPool.java	89.12% <0.00%> (+0.19%)	72.00% <0.00%> (ø%)

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[olavloite]

LGTM.

We should also update the examples in the Javadoc in the DatabaseClient interface (and possibly also other places), but I think we can do that in a separate PR.