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.