Add optional argument for overriding get_tls_context() parameters

[yzhan289]

What does this PR do?

This PR adds the option for get_tls_context() to add an overrides argument to set config values.

This is intended to be compatible with integrations that hardcode the values of SSL context config values in their checks.

Note that the overrides argument takes precedent over reading the values for the respective config value.

This change can affect

• http_check: https://github.com/DataDog/integrations-core/blob/master/http_check/datadog_checks/http_check/http_check.py#L309-L311
• proxysql: https://github.com/DataDog/integrations-core/blob/master/proxysql/datadog_checks/proxysql/ssl_utils.py#L17.

This can also be applied to recently updated SSL context checks, such as

• tls: https://github.com/DataDog/integrations-core/pull/8230/files#r550231989
• vertica: https://github.com/DataDog/integrations-core/pull/8228/files#r550234204.

Motivation

Some older integrations specifically set values for their SSL context without reading the config values. However the get_tls_context() function abstracts the creation of the SSL context away without allowing the check to manually set config values that should stay the same. This could affect the intended functionality of the check while lacking visibility.

Additional Notes

SSL context and TLS context in my other PRs are referring to the same things.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached