move authenticate dispatch to multimethod for external dispatch

[pyncc]

In support of adding a small extension to vault-clj, open up authenticate! dispatching.

Eg, an external library could implement vault.client.http.authenticate-type! for aws instance auth with:

(:require '[big-annoying-aws-library :as aws])

(defmethod vault.client.http.authenticate-type! :aws-iam
    [client _ credentials]
    (let [{:keys [iam-role] credentials]
        (aws/do-instance-authentication-hoop-jumping iam-role))

[codecov-io]

Codecov Report

Merging #28 into develop will increase coverage by 0.36%.
The diff coverage is 86.66%.

@@             Coverage Diff             @@
##           develop      #28      +/-   ##
===========================================
+ Coverage    27.22%   27.58%   +0.36%     
===========================================
  Files            6        6              
  Lines          584      580       -4     
  Branches        17       17              
===========================================
+ Hits           159      160       +1     
+ Misses         408      403       -5     
  Partials        17       17

Impacted Files	Coverage Δ
src/vault/client/http.clj	20% <86.66%> (+0.53%)	⬆️

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[greglook]