feat: add sessionLimitOverride always record by type

[williazz]

Purpose

If merged, RUM users can use the below example to bypass sessionEventLimit and record whatever events they need. However, this PR is limited to "always record" configuration and does not support specific limits, which would need us to migrate event counts from cookies to localStorage to avoid pollution.

const config: AwsRumConfig = {
    ...
    sessionEventLimitOverride: {
        'com.amazonaws.rum.http_event': 0, // always record
        'custom_type': 1 // non-zero limits are not supported in this PR, and will be handled by sessionEventLimit
    }
}

Documentation updates will be addressed in a separate PR.

Depends on

#504

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Engineering plan for #480 summarized here #480 (comment)

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.