Support MFA for :password in :sentinel option

[ahovgaard]

This PR makes it possible to specify a common :password option for sentinels as an MFA. For example:

Redix.start_link(
  sentinel: [
    group: "mymaster",
    sentinels: [
      [host: "localhost", port: 26379]
    ],
    password: {System, :fetch_env!, ["PASSWORD"]}
  ]
)

Currently, the schema in Redix.StartOptions only allows this option to be a string, even though it allows to use an MFA for the :password option in each individual sentinel in the :sentinels list. It already works in the implementation, however, so this PR simply expands the NimbleOptions :type for the option and adds additional test cases.

[coveralls]

Pull Request Test Coverage Report for Build 493f62b931f23edc45564e7c9196902e6d5d6889-PR-257

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 86.003%

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Totals
Change from base Build 3a913079b5be8736e50cf83e7a0172fcfb44ee46:	0.1%
Covered Lines:	596
Relevant Lines:	693

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💛 - Coveralls