fix: reuse AsyncInitializationWrapper in SpringDelegatingLambdaContainerHandler

[deki]

Issue #, if available:
#758

Description of changes:
Did some testing with this app https://github.com/aws-samples/aws-lambda-java-workshop/tree/main/labs/unicorn-store/software/unicorn-store-spring and 512 MB memory.
Comparison between
old StreamLambdaHandler implementation: Duration: 648.79 ms Billed Duration: 649 ms Memory Size: 1024 MB Max Memory Used: 333 MB Init Duration: 9747.09 ms
and
new SpringDelegatingLambdaContainerHandler Duration: 16626.82 ms Billed Duration: 16627 ms Memory Size: 1024 MB Max Memory Used: 330 MB Init Duration: 1344.65 ms

With this change the init is done properly. Threading still needs to be removed from SCF.

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[maschnetwork]

LGTM

[olegz]

LGTM as well