Add pbkdf2-sha256 as a FIPS 140 approved password hashing algorithm

[iAbhishek91]

Summary

Closes #2294.

password_hashing.algo currently only supports bcrypt and
argon2id, neither of which is a NIST/FIPS 140 approved algorithm.
This prevents building a FIPS-compliant SFTPGo image, since there's
no FIPS-approved algorithm to select even with a FIPS-enabled Go
toolchain.

This PR adds pbkdf2-sha256 (PBKDF2-HMAC-SHA256, per NIST SP 800-132)
as a third selectable value for password_hashing.algo, plus a new
password_hashing.pbkdf2_options.iterations setting (default
600000, minimum enforced 10000 per NIST SP 800-132).

SFTPGo already implements PBKDF2 verification, used for migrating
password hashes from other systems (comparePbkdf2PasswordAndHash,
supporting $pbkdf2-b64salt-sha256$, $pbkdf2-sha256$,
$pbkdf2-sha512$, $pbkdf2-sha1$). This change reuses that existing
verification path for the new pbkdf2-sha256 creation option, instead
of adding a parallel implementation.

Changes

• internal/dataprovider/dataprovider.go: add HashingAlgoPBKDF2SHA256
  constant and Pbkdf2Options config struct; hashPlainPassword now
  creates a $pbkdf2-b64salt-sha256$<iterations>$<salt>$<hash> hash
  when selected, using util.GenerateRandomBytes for the salt and the
  existing pbkdf2SHA256B64SaltPrefix format so it round-trips through
  the existing comparePbkdf2PasswordAndHash verifier unchanged;
  initializeHashingAlgo validates the configured iteration count.
• internal/config/config.go: default pbkdf2_options.iterations to
  600000 and register the corresponding viper default.
• internal/dataprovider/admin.go, apikey.go, share.go: these
  duplicated the bcrypt/argon2id branching for hashing admin passwords,
  API keys and share passwords. Replaced with a call to the shared
  hashPlainPassword helper so all four credential types pick up
  pbkdf2-sha256 consistently, and added the matching
  comparePbkdf2PasswordAndHash branch to each type's verification
  path (previously only dataprovider.go's user-password verification
  handled pbkdf2 hashes; admin/API key/share verification assumed
  bcrypt-or-argon2id).

This is intentionally scoped to just the hashing algorithm; it doesn't
touch transport/TLS crypto, which is a separate FIPS concern already
addressed by building with a FIPS-enabled Go toolchain
(GOEXPERIMENT=boringcrypto), as noted in #1272.

Test plan

• go build ./...
• go vet ./internal/dataprovider/... ./internal/config/...
• gofmt -l on all changed files (no output)
• Manually verified hash/verify round-trip for pbkdf2-sha256
  (correct password matches, wrong password is rejected, hash has
  the expected $pbkdf2-b64salt-sha256$ format) with a local test;
  removed before submitting since this package has no existing
  test file to extend
• Would appreciate maintainer guidance on whether a
  dataprovider_test.go should be introduced for this, since none
  currently exists in this package

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[drakkan]

see #2294 (comment). Thanks anyway