Add Userpass auth method

[brendonjwong]

Implemented the Userpass's login method. Validated with the added integration test for userpass.

This should resolve https://github.com/amperity/vault-clj/issues/78.

[brendonjwong]

The linting failure seems unrelated to these changes?

src/vault/secret/kv/v1.clj:157:33: error: Unresolved symbol: random-uuid
src/vault/secret/kv/v2.clj:556:40: error: Unresolved symbol: parse-long
src/vault/secret/kv/v2.clj:568:23: error: Unresolved symbol: random-uuid
linting took 404ms, errors: 3, warnings: 0

Exited with code exit status 3

[codecov-commenter]

Codecov Report

:exclamation: No coverage uploaded for pull request base (vault-2.x-rewrite@df04c91). Click here to learn what that means. Patch has no changes to coverable lines.

:exclamation: Current head 31d746e differs from pull request most recent head 094bdae. Consider uploading reports for the commit 094bdae to get more accurate results

Additional details and impacted files

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