[GEP-26] Add support for web identity authentication to S3 snapstore

[vpnachev]

How to categorize this PR?
/area backup security ipcei
/kind enhancement

What this PR does / why we need it:
Add support for web identity authentication to S3 snapstore

Which issue(s) this PR fixes:
Part of gardener/gardener#9586

Special notes for your reviewer:
cc @dimityrmirchev

Release note:

The container base image has been updated to debian12. 

S3 snapstore now support web identity authentication.
To be enabled, the `roleARN` and `token` credentials files or json fields must be set, while `accessKeyID` and `secretAccessKey` has to be unset.

[dimityrmirchev]

Thanks! I left some small comments/questions.

[vpnachev]

@gardener/etcd-druid-maintainers kindly review this PR

[dimityrmirchev]

/lgtm

[ishan16696]

/assign

[ishan16696]

few nits:

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[ishan16696]

LGTM!!