feat: allow dynamic kerberos lease TTL [Issue #45]

[r-ushil]

fixes #45

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[r-ushil-gsa]

Just wanted to bump this, as it's been waiting for review for a while, passes the tests and doesn't seem too intrusive of a change! @hc-github-team-secure-vault-ecosystem

Thanks again!

[fairclothjm]

@r-ushil Hello, thanks for the PR! Do you mind adding some tests?

[r-ushil]

should be done. @fairclothjm

bats /home/rushil/vault-plugin-auth-kerberos/test/acceptance/server-enterprise-basic-tests.bats
server-enterprise-basic-tests.bats
 ✓ auth/kerberos: create namespace
 ✓ auth/kerberos: register plugin
 ✓ auth/kerberos: enable and configure auth method
 ✓ auth/kerberos: setup and authentication within a Vault namespace
 ✓ auth/kerberos: dynamic TTL feature