okta: allow configuration of http keep-alives

[efd6]

What does this PR do?

Allows configuration of HTTP keep-alives. The default used departs from the current behaviour (disabled) to ensure the best experience for most cases by allowing connection reuse, but can be reverted to the old behaviour if needed.

It is an advanced configuation option since it's expected to not be needed in the vast majority of cases.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Closes okta: add configuration options to allow users to make use of httpjson keep-alive options #4844

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• Start Time: 2023-03-09T01:21:14.719+0000

• Duration: 14 min 2 sec

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Passed	7
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Total	7

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Name	Metrics % (covered/total)	Diff
Packages	100.0% (1/1)	💚
Files	100.0% (1/1)	💚
Classes	100.0% (1/1)	💚
Methods	100.0% (19/19)	💚 9.091
Lines	87.864% (543/618)	👎 -4.929
Conditionals	100.0% (0/0)	💚

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Package okta - 1.16.0 containing this change is available at https://epr.elastic.co/search?package=okta